Permanent effects of postnatal gonads on dopamine (DA)-mediated regulation of PRL secretion were examined in adult male rats of the Wistar-Imamichi strain. Rats were bilaterally orchidectomized at varying postnatal days, i.e. 8 h after birth [neonatal castration (NC)] or at 1, 2, 3, 4, or 6 weeks of age, and subjected to experiments at the age of about 10 weeks. Ten-week-old intact adults were also examined. All groups were injected ip with saline or sulpiride (1 mg/kg BW), and blood samples were collected by rapid decapitation 20 min later. Anterior pituitaries (AP) thus obtained were subjected to determination of PRL content. The circulating level of testosterone was similar among all castrated groups. Rats castrated on or after 3 weeks of age showed a significantly smaller PRL response to sulpiride than intact adults. In turn, sulpiride-induced PRL release in rats castrated on or before 2 weeks of age was similar to that in intact adults. However, the AP PRL content of saline control was uniformly lower in castrated males than in intact adults regardless of age at castration (P less than 0.01). The value in males castrated at 1 week was 2.0-fold higher than that in NC males, but a decreasing trend was observed in groups castrated at 2 or 3 weeks. That such a stimulation of AP PRL content by the neonatal testis is not mediated by an action of estradiol converted from testosterone has been suggested by the failure of daily neonatal tamoxifen (estrogen antagonist) treatment of intact males, before castration at 1 week, to decrease AP PRL content during adulthood. Sulpiride-induced reduction of the AP PRL content was significant only in males castrated at 6 weeks and intact adults. Characteristics of AP DA receptors labeled by [3H]spiperone were not different between males castrated at 6 weeks and intact adults. However, a heterogeneity was observed between NC males and a set of males castrated at 6 weeks and intact adults. The Kd was significantly (P less than 0.05) greater in NC males (mean +/- SE, 1.19 +/- 0.22 nM) than in males castrated at 6 weeks (0.52 +/- 0.08 nM), and the number of binding sites was significantly (P less than 0.05) larger in NC males (20.4 +/- 1.6 fmol/pituitary) than in both males castrated at 6 weeks and intact adults (12.6 +/- 2.0 and 13.1 +/- 1.9 fmol/pituitary, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)