Injections of increasing doses of testosterone propionate (TP) to male rats for the first 30 days of postnatal life (0.5mg, 1mg and 2mg in 3 successive 10-days periods, respectively) brought about an inhibition of spermatogenesis and a marked atrophy of reproductive accessory glands. These changes persisted for 4 months after the treatment was discontinued. However, the effects of a single (0.5mg on the day of birth) or successive injections of TP for the first 10 days of life (0.5mg/day) were less marked.