The pharmacokinetics of the antitrypanosomal drug isometamidium were studied in lactating goats after intravenous and intramuscular administration at a dose of 0.5 mg/kg body weight, in a crossover design at an interval of 6 weeks. Following intravenous administration, the half-life of the disappearance of the drug from plasma during the terminal phase was 3.2 h, and the mean residence time was 2.4 h. The apparent volume of distribution averaged 1.52 l/kg, and the mean total body clearance was 0.308 l/kg/h. After intramuscular administration, the absolute bioavailability was low, averaging 27%. This was consistent with a low mean maximum concentration of 24 ng/ml which occurred after 6 h. No drug was detectable (less than 10 ng/ml) in milk samples collected over a period of 14 days following drug administration by either the intravenous or intramuscular route. In tissues analysed when the goats were killed 6 weeks after administration of the second dose, no drug was detectable (less than 0.4 micrograms/g wet tissue) in the liver, kidney and muscle. However, at the injection site, drug concentrations varied from less than 0.4 to 18.8 micrograms/g wet tissue.