The principal aim of this study was to investigate the ontogeny of an opioidergic mechanism mediating the suckling-induced inhibition of LH secretion during lactation in sows. In contrast to an increase in LH secretion in response to naloxone treatment on days 10 and 11 of lactation (P < 0.05), a single injection of 2 mg naloxone kg-1 at 39, 51, 63, or 75 h post partum had no effect. However, the last of four injections of 2 mg naloxone kg-1 given at 12 h intervals to group IV sows did elicit a positive LH response (P < 0.05). Multiple injections of 1 mg naloxone kg-1 at 3 h intervals over 30 h on day 10-11 consistently increased (P < 0.05) mean plasma LH with no evidence of induced refractoriness to repeated use of the antagonist. Similarly, naloxone did not affect mean plasma prolactin in the immediate postpartum period, but either repeated naloxone treatments on day 10-11 or single naloxone injections on day 10 or 11 of lactation decreased plasma prolactin (P < 0.05). Therefore, the regulation of LH and prolactin secretion in lactating sows changes with time post partum. An opioid-dependent mechanism is an important component of the suckling-dependent regulation of LH and prolactin secretion in established lactation, but not during the first 72 h postpartum period.