1. Metabolites of codeine were determined by use of h.p.l.c. in urine of male mice, rats, guinea pigs and rabbits injected with 10 mg codeine/kg subcutaneously. 2. In 24 h urines of these species, unchanged codeine, codeine glucuronide, free morphine, and morphine-3-glucuronide were as follows: mice, 6.8, 1.6, 0.8 and 7.6% dose; rats, 1.6, 0.2, 4.3 and 23.9% dose; guinea pigs, 1.6, 39.8, 0.2 and 1.6% dose; rabbits, 2.2, 24.5, 1.3 and 17.9% dose. Urinary excretion of morphine-6-glucuronide was 0.7% dose in guinea pigs, 1.9% in rabbits, and not detectable in mice and rats. Norcodeine was found only in the urine of mice. 3. These results indicate that codeine is metabolized in all four species by glucuronidation and by oxidative N- and O-demethylation, but the quantitative excretions of metabolites were quite different in different species.