1. An apparatus is described that allows CO2 expired by monkeys to be trapped and measured and urine and faeces to be separated and collected. 2. When [14C]cloguanamile was administered to rats and to patas monkeys, a marked species difference was seen in the proportion of 14CO2 expired. 3. Suppression of the gut flora by neomycin revealed that this difference was due to enterofloral metabolism.