Buspirone (BP), a newly developed antianxiety agent, forms 1-(2-pyrimidinyl)-piperazine (PmP) during its biotransformation in rats and man. After oral administration of pharmacologically effective doses of BP-hydrochloride to rats (1 and 10 mg/kg), the metabolite appears in significant amounts in body fluids and tissues; it is highly concentrated in the central nervous system, the brain-to-plasma concentration ratios being approximately 5 at the time of the maximum concentrations (Cmax). In man given the anxiolytic dose (20 mg) of BP the metabolite reaches higher plasma Cmax values than its parent drug. Its plasma elimination t1/2 is more than double that for BP. These results, together with the fact that PmP is biochemically and pharmacologically active, suggest that the metabolite may contribute significantly to the central effects of the parent drug.