This study reports the endocrine effects of chronic methadone (METH) administration. Two treatment regimens were tested: a constant (5 mg/kg/day, CD) or increasing dose (5 mg/kg twice daily, increasing 1 mg/kg/day, ID). Basal hormone levels and endocrine responses to opiate challenge were determined on days 5, 10 and 20 and after withdrawal. METH effects on hormone secretion varied with treatment duration, dose and hormone. Tolerance to METH effects on corticosterone (CS), prolactin (PRL), growth hormone (GH), thyroid-stimulating hormone (TSH) and luteinizing hormone (LH) developed during the ID regimen and basal CS, TSH and LH levels were altered. In addition, serum testosterone, T3 and T4 decreased significantly during this regimen. In contrast, only CS secretion was affected markedly by the CD regimen. Resting levels were elevated by day 5 and the CS response to acute METH challenge was reversed. Tolerance also developed to METH-induced TSH suppression, but only with longer treatment. Similarly, basal TSH and LH levels were affected only with longer treatment. Basal PRL, GH and LH levels and LH, PRL and GH responses to acute METH challenge were not affected by the CD regimen. Changes in basal CS and TSH levels observed in these studies probably reflect abstinence, as even more pronounced changes occurred after naloxone-precipitated abstinence and suppressed rather than reversed responses were observed if animals were withdrawn for 36 hr before testing. Tolerance in some endocrine systems appears to be quite long-lasting, as CS, TSH and PRL responses to METH challenge were still decreased 3 weeks after withdrawal from the ID regimen.(ABSTRACT TRUNCATED AT 250 WORDS)