Samples of urine and feces were collected daily from a normal human volunteer who had received a dose of pseudoracemic phenprocoumon [an equimolar mixture of (R)-[12C]- and (S)-[2-13C]phenprocoumon] containing a tracer dose of 10 microCi of [14C]phenprocoumon and analyzed by TLC, HPLC, and GC-MS. After 25 days, 96% of the radiolabeled material was recovered (62.8% in urine and 33.3% in feces). By isotopic dilution and comparison to the Rf values, retention times, and mass fragmentograms of synthetic standards, the metabolites of the drug were identified as the 4'-, 6-, and 7-hydroxy analogues of phenprocoumon. Virtually all of the recovered radioactivity could be accounted for by the parent drug (approximately 40%) and the three metabolites (approximately 60%). The formation of both 4'-(8.1% of administered dose) and 7- (33.4% of administered dose) hydroxyphenprocoumon was highly stereoselective, giving S/R ratios of 2.86 and 1.69, respectively. The formation of 6- (15.5% of administered dose) hydroxyphenprocoumon showed little stereoselectivity (S/R ratio equal to 0.85). The urinary excretion pattern was also confirmed in four additional healthy male subjects who received a single oral dose of pseudoracemic phenprocoumon and whose urine was analyzed by GC-MS. All the drug-related materials (both hydroxylated metabolites and parent compound) that were excreted into the urine were extensively conjugated.