Chlorohydrins U-5897 (3-chloro-1,2-propanediol) and U-15,646 (1,1′-(pentamethylenedioxy) bis [3-chloro-2-propanol]) are antifertility compounds in the rat; the former is also effective in the guinea-pig. U-5897 is five times as potent as U-15,646. Fertility is lost in less than a week by daily oral or subcutaneous administration and regained within 1 week post-treatment. Treated males show normal libido and when mated produce vaginal plugs; uterine and oviducal sperm numbers, morphology, and motility are indistinguishable from controls. A 7-week treatment with twice the minimal effective dose (MED) of U-5897 caused no change in general health, mating, spermatogenesis, or epididymal sperm motility. Five times the MED will produce lesions in the caput epididymidis. The antifertility effect is related somehow to the inability of spermatozoa to fertilize ova as none recovered was cleaved. A threshold dose of U-5897 allowed several implants to develop but they resorbed before term. A sub-threshold dose (U-15,646 for 6 weeks or U-5897 for 8 days) produced no change in reproductive capacity or sex ratio of offspring. These compounds may produce changes in the vasculature of the epididymis which could hinder sperm maturation in the caput epididymidis and lead to inability to fertilize ova.