1. Single oral doses of the anticholinergic drug [14C]Sormodren to rats (1 mg/kg), dogs (0.3 mg/kg) and humans (0.03 mg/kg) were well absorbed. Excreted in urine and faeces were means of 31 and 70%, 53 and 39%, and 78 and 4% in rats, dogs, and humans, respectively, during five days: excretion was prolonged and still incomplete at five days in humans. 2. Peak plasma levels of 14C (scaled for dose) were generally reached within 1-2 h after oral doses in rats, 49 (ng/ml)/(mg/kg), dogs 290 (ng/ml)/(mg/kg) and humans 410 (ng/ml)/(mg/kg), and declined with half-lives of approx. 5, 12 and 30 h, in these species respectively. Repeated oral doses of [14C]Sormodren to dogs resulted in some accumulation of 14C in the plasma. 3. Tissue concn. of 14C in dogs were generally higher than those in rats, particularly in the brain, lungs and eyes. The tissue distribution of 14C in rats and dogs was consistent with that of a compound readily eliminated by both renal and hepatic routes. 4. Basic metabolites in dog and human, urine and plasma were investigated using a combination of h.p.l.c. and g.l.c.-mass spectrometry. Unchanged Sormodren was not detected in the dog samples and was only a minor component in human urine and plasma. Some metabolites were present as conjugates. 5. A basic extract of enzyme-hydrolysed dog urine (5 mg/kg dose) contained 42% of the urine 14C. The major metabolites in this fraction were identified as three isomers of monohydroxy-N-desethyl-Sormodren and three isomers of monohydroxy-Sormodren, resulting from hydroxylation in the bicyclic ring. The positions of oxidation were not determined. A similar extract from dog urine (0.3 mg/kg dose) contained 26% of the urine 14C and the major metabolites were identified as isomers of monohydroxy-N-desethyl-Sormodren. 6. A basic extract of enzyme-hydrolysed human urine (0.03 mg/kg dose) contained 23% of the urine 14C. The unchanged drug was only a minor component and most of the radioactivity was associated with five isomers of monohydroxy-Sormodren, hydroxylation having occurred in the bicyclic ring. 7. Basic extracts of dog and human plasma only contained about 10% of the plasma 14C. Metabolites were chromatographically similar to the hydroxylated metabolites identified in the corresponding urine samples.