occurred between 24 and 36 hours after dosing. The other three sizes generally reached maximum concentrations in feces at 36 to 64 hours after dosing. Percent of Dose Retained vs. Time Although considerable variation in excretion rates was observed among the four cows for the various sized particles, slower excretion rates were consistently associated with the larger particles. The average percent of dose retained for the various particle sizes is given in Table 4 as a function of days after dosing. An indication of variability among cows is noted by the standard deviations. Data for the first 7 days, while the cows were in metabolism stalls, are shown in Figure 1. The three larger size particles tend to be excreted at similar rates, while the smallest particles are excreted much more rapidly. Data for each individual cow exhibited this tendency. However, as indicated in Table 4, the excretion rates for the three larger sized particles varied widely among the individual cows. The average half-times for fecal excretion are given in Table 5. The half-times for particle excretion were obtained from the slopes of the best fit lines shown in Figure 1. These data indicate the excretion rate after excretion starts and neglect the initial period of retention from ingestion of initial excretion. While not of particular interest in this experiment, these data are included to facilitate comparison of results with those of other investigators. Such comparisons are given in the discussion section of this report. The variability in excretion rates is obvious from the range of values for each particle size. Average rates were determined from a computed exponential best fit for all data points from the four cows during the 7 days in metabolism stalls. Results of analyses on urine, blood, and milk showed that no significant in vivo leaching of the radionuclides from the particles occurred. The detection limits in these samples were less than 107°% of the administered dose. An important finding in this experiment was that essentially all the particles were recovered in feces. This suggests that very little of the sand was trapped in the fine structures of the GI tract. Average half-time data from Table 5 are plotted vs. particle size in curve 1 of Figure 2. This information can be used to make an initial estimate of dairy cow excretion rates for ingested particles. However, the information on variability among animals, Tables 4 and 5, should also be considered when making such estimates. Percent of Dose Retained vs. Fecal Output Intuitively, it would appear that the particle elimination rate should be directly influenced by the fecal production rate for the cows. Data in Table 6 suggest little relationship between cumulative fecal output and the percent of particles excreted. 156