1. After administration of [phthalidyl-14C] talampicillin (Talpen) to rat, dog and man, radioactivity was excreted mainly in the urine (90%, 86% and 98% in rat, dog and man respectively). 2. After administration of [ampicillin-14C] talampicillin, radioactivity was excreted in the urine of rats and dogs to a lesser extent (35% in both species) and only a small proportion of the dose was excreted in the bile (6% in rats, less than 0.1% in dogs). 3. The pattern of radiometabolites was very similar in extracts of the urines of radiometabolites was very similar in extracts of the urines of rat, dog and man dosed orally with [phthalidyl-14C]talampicillin. The major metabolite was 2-hydroxymethylbenzoic acid. 4. Unchanged talampicillin was present in the hepatic portal vein blood of dog and thus reached the liver, whereas in rat, no parent compound could be detected in portal vein blood. This result may help to explain differences in toxicity of the compound in rat and dog. 5. Studies in vitro showed that the intestinal wall is an important site of hydrolysis of talampicillin in rat and dog.