Previous studies indicate that 7 days after testicular administration of naloxone, serum testosterone (T) concentration and basal T secretion in vitro decrease significantly. In neonatal rats, short-term (2 h) intratesticular treatment with 0.3 µg (D-Met2-Pro5)-enkephalinamide suppresses steroidogenesis. In the present study, testicular treatment with naloxone or enkephalinamide was combined with hemivasectomy (which also includes transection of the inferior spermatic nerve). When local treatment with naloxone was combined with vasectomy in 15-day-old rats, the opioid antagonist-induced increase in testicular weight, the decrease in basal T secretion of the treated gonad and the drop in serum T level could not be observed 7 days postsurgery, despite the fact that in immature rats, hemivasectomy, by itself, also reduces steroid secretion. Similarly, in 5-day-old animals, the short-term (2 h) effect of testicular enkephalinamide on T secretion was prevented by vasectomy. These data suggest that local testicular actions exerted by endogenous opioid peptides might be modulated by the inferior spermatic nerve