Plutonium studies outlined in this paper are somewhat different from many transuranic metabolism projects in that, almost irrespective of nuclide concentration, the most critical materials are milk, liver, eggs and skeletal muscle. Milk and eggs are especially important materials since their production represents an efficient way of converting dietary crude Protein and energy into edible products. Conversion efficiencies are probably of even more significance For future generations which may not have the same agricultural operations that Americans enjoy today. Once nuclide transport via edible anima] products has been confirmed, some further and perhaps more subtle problems become evident. Two such problems are recycling and biological availability. In beef cattle, approximately 44 percent of the live animal weight is not edible, but substances such as bone meal and blood are occasionally used as a protein concentrate for animal feed. Another recycling problem is introduced through the practice of using poultry and cattle manures in some ruminant diets. Recycling problems may be of marginal importance today since grain is usually used as a feed supplement, but they will probably become more important in the future. The second major question concerns the biological availability of plutonium once it has been incorporated into an edible animal product. Gastrointestinal uptake of plutonium received considerable attention in several early works. In a metabolism study on rats, Scott et al., (1948) observed that the average value for gastrointestinal absorption of plutonium using three different valence states was approximately 7.0 x 107° percent. Comparative studies on the intestinal uptake of plutonium nitrate revealed no significant differences in total absorption between the rat and pig (Weeks et al., 1956). These results were quantitatively similar to those obtained in a rather extensive report by Katz et al., (1955) which, following a chronic oral plutonium treatment to rats, presented the mean gastrointestinal absorption and retention value at 3.0 x 1073 percent of the administered dose. However, animal age, nutritional status and the relative nuclide availability from in vivo plutonium-labeled food have, in some instances, affected the amount of plutonium absorbed. Ballou (1958) reported that plutonium absorption in day-old rats was 85 times that in the adults and that the amount absorbed dropped abruptly in 21-dayold rats to near adult levels. While investigating the effects of plutonium on mice treated in utero, Finkel (1947) discussed the relative concentrations transported across the placenta or through the milk following parturition. Further studies (Finkel and Kisieleski, 1976) have suggested that the gastrointestinal absorption of plutonium was increased (3.2 percent of dose retained in 18-day-old rats) when milk labeled in vivo with plutonium was administered. Although a fourfold increase in plutonium uptake was noted in iron-deficient mice (Ragan, 1975), iron deficiency would probably not be prevalent at the domestic animal level. Iron requirements of the laying hen, for example, are large in proportion to maintenance requirements, but most domestic animals receive rations of high nutritive value. However, varying degrees of iron deficiency and irregular dietary regimes do exist in the human population and the relative hazards of plutonium-labeled food might be affected by an indivi4ual's nutritional status. 436 This paper will provide a brief overview of some recent plutonium studies using domestic animals and will attempt to direct attention toward selected areas of the food-production cycle where further investigations might prove beneficial. The early experiments of McClellan et al., (1962) and Sansom (1964) have been referenced in previous reports and a comprehensive review on plutonium in biological systems, ranging from large biomes to molecular transformations within individual cells, has already been prepared by Muilen and Mosley (1976). METHODS AND MATERIALS Plutonium metabolism experiments at this Laboratory have used Leghorn hens, Toggenberg goats and Holstein cows as research animals. Hereford cattle are used in a separate effort for grazing studies conducted on a contaminated range at the Nevada Test Site. The largest number of individual metabolism studies used dairy cows. While no metabolism studies were done using beef cattle, it should be remembered that a significant percentage of beef comes from cows and bulls discarded from dairy herds. Furthermore, among dairy breeds, Holsteins produce the best carcass for beef purposes. A fundamental objective of these metabolism studies was to determine what fraction of a quantified exposure would reach the edible animal products. Specific collection and analytical procedures as well as more detailed experimental objectives have been discussed previously (Potter et al., 1971; Stanley et al., 1974; Sutton et al., 1976b; Mullen et al., 1976). Basically samples collected during the studies were analyzed for plutonium-238, based on the 17 keV x-ray from the plutonium isotope. The samples were counted using a phoswich detector containing a thin NaI scintillator backed by a thick CsI scintillator. Overall measurement error was assessed by considering potential uncertainties in the sampling and analytical scheme (Sutton et al., 197@; Mullen et al., 1976). Those samples found to contain low plutonium concentrations were subsequently prepared for analysis by other laboratories which employed more sensitive assay techniques (ashing, extraction, electroplating and counting with an alpha spectrometer). RESULTS AND DISCUSSION Summary results from previous studies are presented for dairy cows (Tables I and 11), goats (Table III) and chickens (Table V). As noted in Table I, total plutonium transport to bovine milk following oral exposure was not great and, on a percentage-of-oral-dose basis, was observed to be 2 x 1074 and 2 x 1075 following citrate-buffered plutonium nitrate and plutonium dioxide treatments, respectively (Stanley et al., 1974). In some cases, total muscle activity closely approximated the total liver activity but was, of course, based on a much greater mass of tissue. 437