Treatment of rats with the serotonin 5-HT1A agonist 8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT, 1 mg/kg s.c.) markedly elevated plasma levels of corticosterone (CORT), adrenocorticotropic hormone (ACTH) and prolactin (PRL); the levels of growth hormone were unaffected. Pretreatment with the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 6 mg/kg s.c.) greatly attenuated the increase in plasma CORT produced by 8-OH-DPAT (0.3 mg/kg s.c.). Prevention of EEDQ-induced 5-HT1A receptor inactivation by prior treatment with the reversible mixed 5-HT1A/beta-adrenergic antagonist (+/-)pinodolol (30 mg/kg s.c.) blocked the reduction of the CORT response to 8-OH-DPAT. In contrast, prevention of EEDQ-induced inactivation of 5-HT2, alpha-1- and alpha-2-adrenergic and D1 and D2 dopamine receptors by a cocktail of selective antagonists of these receptors did not block the attenuation of the CORT response to 8-OH-DPAT. Dose-response curves were obtained for 8-OH-DPAT (0.01-3 mg/kg s.c.)-induced elevation of plasma CORT, ACTH and PRL after treatment (24 hr earlier) with vehicle or EEDQ (6 mg/kg s.c.) and analyzed for the extent of receptor reserve. Whereas substantial receptor reserves were observed for the 8-OH-DPAT rise in plasma CORT (80%) and ACTH (50%), no receptor reserve was seen for the increase in plasma PRL. The results are discussed with regard to potential differences in the receptors, G proteins, effectors and/or stoichiometric relationships between these components of the signal transduction pathway, leading to elevation of these plasma hormones after treatment with 8-OH-DPAT.