Dairy cattle ingest moderate amounts of particulates during normal foraging. These particles are ingested after becoming deposited on forage plants by air currents, or because they are associated with plant roots ingested by cattle. In addition, soil that becomes deposited on the snouts of cattle is licked off and ingested. The gastrointestinal absorption efficiency (solubility) of material in the particles is related to the residence time of particles in the gut, and the residence time could vary for different sizes of particles which the animal may ingest. Transuranic elements are usually in almost insoluble chemical forms when found However, it has been shown that relatively insoluble in environmental media. forms of plutonium are solubilized to a considerable degree in the gastrointestinal environment of cattle (Barth, 1975; Stanley et al., 1975). Information on the gastrointestinal passage rates for various soil particle sizes is, therefore, relevant to the potential absorption efficiency for substances entrained in debris ingested by animals grazing in contaminated areas. MATERIALS AND METHODS Dose Material The isotopes used as tracers were selected on the basis of noninterfering gamma~ray energies in order to facilitate analyses. The silica sand particles were sized and labeled with radionuclides at the Stanford Research Institute, Menlo Park, California (Lane, 1971). The radioisotopes were fused onto the sand and the particles were washed in dilute hydrochloric acid (pH of 1) prior to dose preparation. Less than 1% of the activity was removed by the acid wash. The particles appeared to be nearly spherical upon microscopic examination. Each of four size ranges of particles was labeled with a specific radionuclide as a tracer for a unique particle size. The labeled particles and associated parameters are described in Tables 1 and 2. The sand was weighed into gelatin capsules and administered orally via a balling gun. Each cow received individual doses of all four sand sizes. Weighed aliquots of each dose batch were used in preparation of gamma counting standards. Facilities The study was performed at the experimental farm operated by the U.S. Environ- mental Protection Agency at the Nevada Test Site. The cows were maintained in ‘metabolism stalls located in a specially designed metabolism room. Water and hay (approximately 20 kg/day) were provided ad libittwn. Pelleted dairy ration (2.3 kg/feeding) was provided at each milking, morning and evening. Animals were placed in metabolism stalls 24 hours before dose administration. An indwelling inflatable catheter was used for separating urine from feces. Fecal samples were collected in a grid-covered pan lined with polyethylene sheeting. The cows were milked morning and evening using a milking machine. Animal descriptions are given in Table 3. 152