A spectrophotofluorometric method has been developed which allows estimation of pentazocine in the urine in the presence of a fluorescent metabolite at drug concentrations as low as 0.2 µg per ml. Urinary excretion of the drug was followed for 24 hours after oral and intramuscular doses of pentazocine. Pentazocine was found to be extensively metabolized; less than 13 per cent of the dose appeared in the urine as unchanged pentazocine. Between 12 and 30 per cent was excreted as a glucuronide coniugate, and at least one other unidentified polar metabolite was detected. The maior portion of pentazocine and metabolites was excreted in the first 12 hours. Prolonged side effects noted in 2 subiects did not appear to correlate with the excretion of pentazocine or its metabolites.