Student volunteers (38 of each sex) were exposed unknowingly overnight to the vapour of pheromonally active substances and compared with controls. The substances were either 5 alpha-16-androsten-3 alpha-ol (androstenol, occurring in human underarm sweat, and known to be pheromonally active in pig and man), or a mixture of short-chain fatty acids (occurring in human vaginal fluid, and known to be sexually attractive to the male rhesus monkey). The following morning, the subjects provided information about their social exchanges since rising, by recording on a standardized test diagram the number, depth, duration and direction of initiation, of all verbal exchanges with other individuals. Irrespective of treatment, males returned significantly higher scores than did females for all exchanges and also for some exchanges initiated by other males. Neither exposure to androstenol nor to the fatty acids had any significant effects on any of the scores of males interacting with either sex, nor on any scores of females with other females. However, exposure of females to androstenol, but not to the fatty acids, resulted in significantly higher scores of exchanges with males, in terms of all parameters for all exchanges. Findings are considered in relation to the origin and maintenance across species of pheromonal communication: evolutionary conservation is seen in terms of the utilization of substances that have provided the means of controlling the social milieu.