Diethylcarbamazine (DEC), 0.5 mg/kg, was taken orally by six patients being treated for onchocerciasis. Blood samples were taken at timed intervals for 48 hr and urine and feces collected for 4 days. Plasma and urinary concentrations of DEC and DEC N-oxide were measured by gas-liquid chromatography. DEC appeared to be rapidly absorbed, with a peak plasma concentration of 150 to 250 ng/ml reached in 2 to 3 hr. There was a secondary rise in plasma DEC concentration at 5 to 6 hr in all patients. In contrast to the way the drug is eliminated in rats, in man it was by both renal and extrarenal routes, with small amounts (+/- 10%) being excreted as an N-oxide metabolite. DEC kinetics were also investigated in five normal subjects and the result were much the same. Clinical implications are discussed.