groups were excreted more slowly and with large variations among cows. Up to 168 hrs were required to excrete 50% of the particles and up to 288 hrs to excrete 90%. These times correspond to fecal elimination of about 230 and 400 kg, respectively. The data show that the passage time for soil particles through the gastrointestinal tract of dairy cows varies significantly with particle size. Some of the sand particles which cattle may ingest have gastrointestinal residence times of greater than a week. If the particles should contain relatively insoluble transuranic elements, this period would provide more time for reactions involved in gastrointestinal absorption, e.g., chelation and soluble compound formation, than would be derived from conventional studies carried out in ruminant digestion investigations. 49