Male chacma baboon (Papio ursinus) sexual arousal response as gauged by the coagulated seminal emissions resultant from masturbatory activity was experimentally assessed. Males were initially exposed to an ovariectomized female prior to and during daily oestradiol benzoate treatment. Males were then re-exposed to the same female prior to and during the wearing of a thermoplastic model of a swollen female perineum. A total of eight different colours were sequentially applied to the model. It was only the red model that elicited a response similar to that shown to the oestradiol-treated female. The behavioural data collected from the stimulus female supported the view that it was model colour that determined the differential male response.