1. The metabolism of [14C]nicotine has been studied in four species of animals, rabbit, rat, cat and squirrel monkey, after administration by different routes. 2. Intravenous injection of 4 mug/kg [14C]nicotine every 60 s for 1 h results in peak blood levels of approximately 100 ng/ml in all species but the rabbit. [14C]Cotinine levels in blood vary widely between species. 3. Subcutaneous injection of 0.4 mg/kg [14C]nicotine produces similar peak blood nicotine levels but the time course, for a given species, is different. 4. Intragastric instillation of 1 mg/kg [14C]nicotine to the cat and rabbit results in much lower levels of [14C]nicotine in blood and relatively high levels of [14C]cotinine. 5. Urinary excretion data indicate that, irrespective of route, the squirrel monkey excretes only a small proportion of the dose into urine during the period of experiment, of which the major proportion is [14C]nicotine. The cat, in contrast, excretes a relatively large proportion of the dose during the experimental period though only a minor proportion of the radioactivity is due to [14C]nicotine or [14C]cotinine. 6. All four species are potentially useful for model experiments with nicotine, though metabolism of nicotine by squirrel monkey is most similar to man.