1. The metabolism of chlorpheniramine (I) was examinedin vivoin rats and a human volunteer; in the rats a stable isotope was used.2. In addition to the unchanged drug (I) and theN-demethylated metabolites (II and III), nine further metabolites were identified in rat urine, four of which were also found in human urine. ChlorpheniramineN-oxide (IV), 3-(p-chlorophenyl)-3-(2-pyridyl)propanol (V), 3-(p-chlorophenyl)-3-(2-pyridyl)-N-acetylaminopropane (VII) and 3-(p-chlorophenyl)-3-(2-pyridyl)-propionic acid (XIII) were identified in rat and human urine.3. The hydroxylated metabolites of the pyridyl ring of the unchanged drug, II, V and VII, and the glucuronide of XIII were identified only in rat urine. XIII was found in rat urine as long as 6 days after the last dose.We would like to thank Dr K. Saeki, Kobe Women's College of Pharmacy, for the measurement of the high-resolution mass spectra, and Miss M. Maeda for the measurement of1H-n.m.r.