THE range of chemical agents which can produce sterility in experimental male rodents is gradually increasing. (For recent surveys, see Jackson1,2.) The list includes nitroaromatic compounds, for example, nitrofuranes, nitrothiazoles and nitropyrroles, besides a variety of compounds of the alkylating category. Recently, a nitrothiazole derivative, niridazole (‘Ambilhar’), has been introduced for the treatment of schistosomiasis3,4 and part of its effective toxic action involves damage to the gonads of the male and female parasite. Reversible inhibition of spermatogenesis in mice by this compound has been described5. Here we wish briefly to record the ability of two simple unrelated substances, ethylenedimethanesulphonate (EDS) and hexamethylphosphoramide (HMPA) effectively to inhibit the reproductive capacity of Schistosoma mansoni within the mouse.