Cyproterone acetate (CA) was injected daily in eleven rabbits for 3 weeks at a dose of 20 mg/day, and for a further week at a dose of 40 mg/day. After 3 weeks of treatment, the ejaculation frequency was reduced but other measures of sexual behaviour were not significantly changed. There was no reduction in the fructose concentration of the semen, but the volume of the ejaculates decreased. The vesicular glands from the experimental animals showed histological changes typical of those occurring after castration. It was concluded that CA reduced the activity of at least one of the accessory sex glands as well as sexual behaviour. This lends support to the current hypothesis that the endocrine regulation of rabbit sexual behaviour differs from that of the rat.