Spectrophotometric and thin-layer chromatographic methods for determination of imidocarb in biological specimens are described. Following intravenous injection of imidocarb (2.0 mg/kg) into 3 sheep, plasma concentrations, initially averaging 10.8 microgram/ml, decreased to an average of 1.9 microgram/ml within 1 hour and then to less than 1 microgram/ml within the next 4 hours. When imidocarb (4.5 mg/kg) was injected intramuscularly (IM) into 7 sheep, peak plasma concentrations averaging 7.9 microgram/ml were achieved within 4 hours and then rapidly decreased to 4.6 microgram/ml within the next 2 hours. Plasma values then decayed very slowly by first-order kinetics and trace amounts were still present 4 weeks after treatment. Imidocarb was bound to plasma proteins and the apparent volume of distribution was estimated to be slightly higher than the total body water. The concentrations of the drug in the plasma and in the erythrocytes were approximately equal. Detectable amounts were present in all examined tissues 4 weeks after IM administration Twenty-four hours after IM administration, the highest concentrations were in kidney, liver, and brain. The 14C-labeled imidocarb could be detected in all regions of the central nervous system examined, in the hypophysis, and in the pineal body. Metabolic or biotransformation products were not detected by the methods used. Of the administered IM dose, 11 to 17% was excreted in the urine within 24 hours; thereafter, the excretion rate was low, and detectable amounts were still present in the urine for 4 weeks. Renal clearance of imidocarb was less than glomerular filtration rate, indicating net tubular reabsorption. The relatively high concentration of imidocarb in the bile suggests that the bile is an important route of excretion. High concentrations were also found in the mild of lactating ewes, but the drug could not be detected in the plasma of lambs fed milk from these ewes.