A single dose of 14C-anagrelide (6,7-dichloro-1,5-dihydroimidazo [2,1-b] quinazoline-2(3H)-one monohydrochloride) equivalent to 1 mg free base and containing 100 muCi radioactivity, was taken by five healthy, fasting men. Blood, plasma, urine, and feces were analyzed for total radioactivity. Plasma and urine concentrations of anagrelide were determined, and the urinary metabolite profile was established by high-performance liquid chromatography (HPLC). The drug was rapidly absorbed with peak plasma levels of radioactivity equivalent to 50 ng anagrelide per milliliter at about 1 hr. These levels decreased to less than 10% of peak in 24 hr. Plasma levels of anagrelide peaked at 5 ng/ml at about 1 hr, decreased rapidly during the first 6 to 8 hr, and then declined more slowly, with an estimated terminal elimination half-life of about 3 days. No significant quantities of radioactivity were associated with the cellular elements of blood. Anagrelide was extensively metabolized before elimination in urine. Means of 68% and 72% of the dose were excreted in urine as metabolites in 24 and 144 hr, and 10% of the dose recovered in the feces. Several urinary metabolites were detected by HPLC.