SUMMARY Vaginal secretions were collected by lavage from ovariectomized, oestrogen-treated female rhesus monkeys. Ether extracts of these secretions were applied to the sexual skin of long-ovariectomized, untreated females immediately before mating tests with males. Application of extracts resulted in an immediate and marked stimulation of the sexual activity of males which had been inactive with these females during pretreatment tests when control material was applied. Extracts caused increases in male mounting activity, in ejaculations and in male grooming times, in the apparent absence of any behavioural or genital tract signs of female receptivity. It is concluded from these and other observations that male sex-attractant pheromones, with powerful behavioural effects, are present in ether extracts of oestrogen-stimulated vaginal secretions: this will facilitate their chemical characterization.