of the grazing area from ground zero and grazing intensity. Romney et al. (1975) stated that, as a general rule, the mean fallout particle size decreases with increasing distance downwind from the point of detonation. Hence, a reduced mean particle size is expected in the outer enclosure which would, in turn, result in an increased dissolution rate. Increasing plutonium availabi- lity with decreasing particle size in animals was demonstrated by Bair and Willard (1963) who reported that the rates of pulmonary clearance, translocation, and excretion in both urine and feces were greatest for the aerosol with particles of the smallest median diameters. During the period that these rumen contents samples were collected, the outer enclosure was more intensely grazed than the inner enclosure. Nonfistulated cattle were allowed to graze continually in the outer enclosure, while fistulated cattle were allowed to graze quarterly, for periods of about 4 days, in the inner enclosure. This allowed for regrowth of the edible vegetation in the inner enclosure, while in the outer enclosure, it was continually eaten back. It appeared by visual observation that there were fewer of the fruiting involucres on the Eurotta lanata in the outer enclosure. This suggests a greatly reduced plant surface entrapment of resuspended particulate material, thereby increasing the proportion of plutonium and americium-241, presumably, internally incorporated into the plant tissue. Under these experimental conditions, the effects of distance from ground zero and grazing intensity cannot be differentiated. The amount of soluble plutonium and americium-241 in each digestion phase is shown in picocuries in Table 8. In these trials, the highest amounts of radionuclide in solution, on a radioactivity basis, came from plutonium-239 followed by americium-241. Although on a mass basis minor amounts of pluto- nium-238 were present in the rumen contents collected, on a radioactivity basis the plutonium~238 in solution made up a considerable portion of the soluble plutonium present and in some cases approached or exceeded the levels of soluble americium-241. This can be accounted for by the generally greater solubility of plutonium~-238 in these fluids and the specific activity of plutonium-238, which is approximately 280 times greater than that of plutonium-239 and 5 times greater than that of americium-241. It is emphasized that the data and interpretations presented here and in the earlier report of Barth (1975) apply to the specific field conditions prevailing at the time that rumen contents were collected at Area 13 of the NTS. The field conditions at Area 13, observed to date, that may affect plutonium and americium-241 concentrations in rumen contents and their solubility in the digestive fluids described here are season, predominant vegetative species ingested, vegetative stage of growth, presence of fruiting involucres on Eurotta lanata, grazing intensity, and distance of grazing area from ground zero. 133