Alpha-chlorohydrin (3-chlor-1,2 propanediol) possesses antifertility properties when administered to male rats, hamsters, guinea-pigs, rams and monkeys (Ericsson, 1968; Ericsson & Norland, 1970). The compound is thought to exert its action on the epididymis (Gunn, Gould & Anderson, 1969; Samojlik & Chang, 1970). Coppola (1969) and Gunn et al. (1969) suggested that α-chlorohydrin might act as a metabolic antagonist to glycerol in phospholipid metabolism and the compound has also been shown to act as an alkylating agent in vitro (Jones, Davies, Edwards & Jackson, 1969). The effects of α-chlorohydrin on the adenohypophysis have not so far been reported. The purpose of the present investigation was to compare the cytological changes occurring in the gonadotrophic cells of the anterior lobe of the pituitary in male rats and gerbils after the administration of α-chlorohydrin, with those changes following bilateral gonadectomy. The α-chlorohydrin (sp. gr. 1·326) was supplied by Upjohn Company, Kalamazoo, Michigan, in