was obtained, and each size range was labeled with a gamma-emitting radioisotope. The sand sizes and isotopes used were: Particle Size Isotope Label 15-25 um 74-88 um 1416, 856, 175-246 um 417-495 um 2 4Nin 4856 The particles were washed with dilute hydrochloric acid (pH 1), which removed less than 1% of the activity. Each size range was divided into four portions and placed in gelatin capsules. One capsule (20 g for the three smaller.sizes, 40 g for the largest particles) of each size was orally administered to each of four lactating Holstein dairy cows maintained in metabolism stalls. The cows were fed their normal dairy ration of hay and pelleted food; water was provided ad Libttum. Urine was collected by means of an indwelling catheter. Fecal material from each cow was collected, weighed, obtain a 24-hr composite sample. and mixed to Samples for each cow were analyzed by gamma-ray spectroscopy to determine the content of labeled particles. Samples of urine, blood, and milk were analyzed to make sure the tracer radionuclides did not leach from the particles in vivo. Results of analyses on urine, blood, and milk showed that no signifi- cant in vivo leacning of the radionuclides from the particles occurred, as was intended in the study design, The fecal composition was similar for the four cows, and, although the daily fecal outputs per cow varied by about 25%, the labeled sand excretion did not“appear to be related to the amount of daily fecal output. Although variations were observed among the four cows in the retention times for the various sized particles, longer retention times were consistently associated with the larger particles. The smallest particles (15-25 wm) were excreted rapidly in all cows. Half of these particles were excreted within 36 hrs and 90% within 91 hrs of dosing. These times corresponded to about 40 and 110 kg of feces elimination, respectively. The three larger size particle 48