1. Following in vitro treatment with 12 microM 6-hydroxy-dopamine, 2 microM B-oestradiol, 0.1 microM propranolol and 10 microM cocaine vasa deferentia isolated from young rats (21-23 days old) showed supersensitivity to norepinephrine (NE) compared to those from adult (3 months old) rats. 2. The pA2 values for prazosin were higher in young (9.6 +/- 0.1) than in adult (8.3 +/- 0.1) rat vas deferens, with the slopes of the Schild plots not different from 1.0 (0.78 +/- 0.26 and 1.14 +/- 0.14, respectively). 3. The treatment of young rats with a single dose of testosterone abolished the supersensitivity to NE and the higher affinity for prazosin. 4. We conclude that there is a reduction of neuronal NE uptake and a decrease in the sensitivity to NE in the vas deferens as the rat matures sexually. 5. Testosterone induces a decrease in the sensitivity to NE, probably via an action on the alpha 1-adrenoceptor population and the sympathetic nerve discharge in this organ.