ABSTRACT: The effect of a short pulse therapy with cyclosporine on the production of vasectomy-induced cytotoxic sperm antibodies was studied in Lewis rats. Cyclosporine at a dose of 10 mg/kg was administered once daily for 7 days before and 7 days after vasectomy (group A), for 7 days before vasectomy (group B), and for 7 days after vasectomy (group C). Ten rats were studied in each group for a period of 5 weeks. Ten rats were vasectomized and untreated with cyclosporine (group U). All animals in this group developed high titers of cytotoxic sperm antibodies at the end of the first week after vasectomy (primary antibody response). In contrast, all the treated rats of groups A, B, and C had a normal range of sperm antibody titers. By the third week, six of nine rats in the post-treated group C had significant cytotoxic sperm antibody titers, while the other groups had normal antibody titers. In the fourth week, all the rats of groups A and B continued to have normal sperm antibody titers of ≤16, while most animals in groups C (78%) and U (63%) had significant cytotoxic sperm antibody titers. It is concluded that prevasectomy treatment with cyclosporine may significantly reduce or delay the production of cytotoxic sperm antibodies in Lewis rats.