The administration of 4 oral doses of 200 mg/kg per wk for 2 wk was sufficient to cause a rapid decline in testicular weight and to disrupt spermatogenesis sufficiently to cause sterility. There was a rapid decline in relative testicular weights following drug treatment. 5-Aminoindazole apparently is an active antispermatogenic compound which has secondary effects on epididymal spermatozoa. It induces exfoliation of immature germ cells, a characteristic of other antispermatogenic agents, and produces long-lasting infertility.