Ten derivatives of halo propanediol were tested orally for male antifertility effects in adult Sprague-Dawley rats. Six of these compounds were effective in inducing temporary sterility at a daily dose ranging from 2·5 to 5 mg/rat. When administered over a minimum period of 7 days, these minimum effective doses did not interfere with the mating behaviour and spermatogenesis even during prolonged medication. However, at ten times the minimum effective dose, Compounds III (1-chloro-2,3-propanediol) and VII (1-chloro-2,3-bis[tetrahydropyran-2-yloxy]propane) caused severe testicular damage and irreversible sterility, while Compound X (4-[chloromethyl]-2-pentyl-1,3-dioxolane) did not. Our studies confirm that the presence of an α-chlorohydrin group is essential in halo propanediol derivatives for antifertility effects in male rats.