show an initial appearance of the marker in feces at 12 to 24 hours after feeding, 80% excretion in 70 to 90 hours, and then slow excretion rates with complete excretion in 7 to 10 days. Experiments with other markers, e.g., chromic oxide and rubber or plastic particles, show that indicators are likely to pass at a different rate than the food with which they were ingested (Dukes, 1955). Moreover, it has been found that there is no consistent relationship between the nature of a cow's diet (e.g., straw, hay, or grass) and the length The specific of time it is retained in the rumen (Annison and Lewis, 1959). gravity of ingested particles is directly related to GI retention time, and the longer retention for higher specific gravities appears to be wholly accounted for by longer retention times in the reticulo-rumen, although the cow diet regime (e.g., feeding schedule and size of food particles) strongly influences rumen retention times (Balch and Campling, 1959). A nearly empty rumen has a short retention time for newly added material, and finely divided material tends to "float" through the system in a short time (Balch, 1950). The results from this experiment indicate that the sand administered (specific gravity of about 2.3 and particle weights ranging from about 10 > to 0.2 mg) The was excreted about as would be expected from dyed-food feeding studies. smallest particles appeared to "float" through the rumen, while the larger particles showed some relative delay in the excretion pattern. Plutonium oxide particles have a specific gravity of 11.46 and should be retained longer Plutonium particles or in the rumen than sand particles of comparable size. atoms in the environment may be bound or fused to siliceous materials and be excreted in a manner similar to sand. The smallest particles first appeared in the feces at 8 to 12 hours after dosing, and the highest fecal concentration occurred at about 30 hours after dosing, which is about the same as reported for stained ground-hay particles (Dukes, 1955; Phillipson, 1970) and for small (25- to 30-ym mean size) fallout particles (Potter et al., 1971). The entire dose of these 20-um particles was excreted very rapidly, and the average retention time Table 7, 1.8 days) was about that reported for stained ground-hay (Dukes, 1955; Phillipson, 1970) and for small fallout particles (Potter et al., 1971). The mean retention time for soluble radionuclides in the dairy cow GI tract was reported to be 0.9 days (Osanov et al., 1974), which is about that found for the 20-ym particles in this experiment. The three larger-sized particles, excreted in a similar fashion from the cows, were first detected in feces at 12 to 16 hours after dosing. However, the highest fecal concentrations occurred later than for the smallest particles, at 36 to 64 hours after dosing. An average of 7 to 10 days was required to excrete 90% of the larger particles. These times are similar to those reported for coarse roughage materials (e.g., long hay) fed to cows (Balch and Campling, 1965; Dukes, 1955; Phillipson, 1970). The mean retentton time for 88- to 175-ym simulated-fallout particles was reported as 4.8 days in cattle (Johnson and Lovaas, 1971). This is slightly longer than the 3.2- and 3.9-day average values (Table 7) obtained in this experiment for 80- and 200-ym particles, respectively. 163