We studied the effects of two ovarian steroid treatments that induce proestrous-like surges in LH secretion on responsiveness to morphine sulfate (MS), as measured by induced hypothermic, antinociceptive, behavioral, and LH secretory changes. Ovariectomized rats received no steroids (OVX), 7.5 micrograms estradiol benzoate 2 days before the experiment (EB), or EB and then 5 mg progesterone 48 h later (EBP). MS administration coincided with the steroid-induced LH hypersecretion that occurs in the EB and EBP rats at 1530-1630 h. Serum LH concentrations were determined 30 min after administration of MS. In OVX and EB rats, MS caused a dose-dependent decrease in serum LH, but even 20 mg/kg MS did not alter serum LH during the EBP-induced LH surge. Brain-mediated morphine-induced analgesia was evaluated in the three steroid treatment groups from measurement of latency to pawlick on a hot plate. EB and EBP rats were less responsive than OVX rats to MS-induced antinociception. EB and EBP rats were also less responsive than OVX animals to the spinal cord-mediated analgesia due to MS, as calculated by tail-flick latency. MS-induced hypothermia revealed a responsiveness order of OVX greater than EB greater than EBP. Whereas MS caused a dose-dependent reduction in locomotor activity in OVX and EB rats, EBP rats showed marked hyperactivity at low MS doses and were less responsive to the suppression of locomotor activity at higher doses. These marked steroid-induced changes in MS responsiveness could not be explained by altered pharmacokinetic disposition of morphine. These data indicate that treatment with EBP, which stimulates a preovulatory-like LH surge, decreases the ability of MS to induce hypothermic, antinociceptive, and behavioral responses and abolishes its capacity to suppress LH release. These effects of gonadal steroids were not observed before the LH surge, which suggests that this surge is linked to the decline in MS sensitivity. Further, the diminished response to MS appears to be a function of the magnitude of the LH surge.