Earlier studies from our laboratory have shown that treatments with gonadal steroids which cause a surge of luteinizing hormone (LH) blunt the effects of morphine on LH secretion, locomotor activity, nociception and temperature regulation. The present study was conducted to determine if the growth hormone (GH) response to morphine sulfate (MS) was also affected during steroid-induced LH surges. Adult ovariectomized female rats were primed with estradiol benzoate (EB: -49 h prior to P4 or oil injection) and/or progesterone (P4; 10.00 h). Seven and one half hours after P4 treatment, at the time of the steroid-induced LH surge, the GH response to an intravenous dose of 0.5, 2.0 or 5.0 mg/kg MS was determined. A GH peak response occurred at 15 min after drug administration and was maximal at the dose of 2 mg/kg MS in all groups. Although the timing of the GH rise was not altered in either EB/oil- or EB/P4-treated animals, a significant blunting of MS-induced GH secretion was observed across all doses of MS with a greater reduction being observed in EB/P4-treated animals. In a second study the GH response to opiates was investigated prior to and following the steroid-induced LH surge to determine if the suppression of opiate-induced GH secretion was confined to the time of the preovulatory LH surge. Before the LH surge (2.5 h), a mild suppression of opiate-induced GH secretion was observed only in EB/oil-treated animals. After the LH surge (13 h after P4 administration), morphine-induced GH secretion was similar in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)