Nitrofurantoin sodium and nitorfurazone, in certain critical concentrations, have the ability to immobilize spermatozoa. In this study, this effect was consisitent with both 5:1, and 10:1 dilutions. Histologic examination of the effects of these drugs as intraoperative vas irrigants revealed no apparent tissue toxicity in the guinea pig. Clinically, we found intraoperative vas irrigation with both of these nontoxic drugs to be of value in attaining azoospermia immediately postvasectomy. Therefore, they appear to be useful supplements to vasectomy.