Three healthy male subjects received single 100 mg oral doses of carprofen containing 20 microCi of 14C-carprofen. Venous blood samples were drawn during the first 48 h after the dose and urine and faeces were collected for 120 h. Concentrations of carprofen and its metabolites in body fluids were determined by TLC and mass spectral analysis. After a lag time of 0.3 +/- 0.1 h (mean +/- S.D.), carprofen was absorbed rapidly and peak concentrations in the plasma were reached in 2.7 +/- 1.3 h. The 14C plasma concentrations declined in a biphasic fashion. The mean half-lives of the initial (alpha) and terminal (beta) phases were 1.1 h and 20.6 +/- 6.1 h, respectively. Biotransformation to a glucuronide metabolite appeared to be the major mechanism of carprofen clearance. In 48 h 74.0 per cent of administered radioactivity was recovered in urine and 14.1 per cent was recovered in faeces. A glucuronide of carprofen comprised 85.0 per cent of the radiolabelled compounds in urine. The remaining radioactivity was comprised of parent drug (12.0 per cent) and un unidentified acid-labile conjugate of the parent drug (3.0 per cent). This pattern of metabolism and excretion is different from that in the dog and rat and may explain species differences in drug activity and toxicity.