The compound 6 chloro-Δ6-1,2,α-methylene 17α-hydroxyprogesterone acetate (SH-714) reduces testosterone-induced weight increases in accessory sex organs of rats and exerts an inhibitory effect on the masculinization of female foetuses caused by the administration of testosterone (Neumann & Kramer, 1964). The present experiments describe the effect of SH-714 on the mating behaviour of adult male rats and guinea-pigs. Each male guinea-pig was given weekly tests during which it was allowed access to a receptive female for 10 min.; the different components of the male's sexual behaviour (sniff, nuzzle, mount, intromission and ejaculation) were recorded. The testing and scoring procedures have been described in detail elsewhere (Young & Grunt, 1951). Only those animals whose score remained adequately stable during three tests were selected for experimental use. For most tests ovariectomized females that had been rendered sexually receptive by appropriate hormone treatments were used as partners, but all males were also tested