We have previously found that histamine (HA) is involved in the mediation of restraint- and ether stress-induced release of the POMC- derived peptides ACTH and β-endorphin (β-END). In the present study we investigated the possible involvement of hypothalamic histaminergic neurons in the mediation of insulin/hy- poglycemia-induced release of ACTH and β-END in conscious male rats. To do so, hypoglycemia stress was performed during 1) inhibition of HA synthesis, 2) activation of inhibitory presynaptic HA H₃-auto- receptors, or 3) blockade of postsynaptic HA H₁- or H₂-receptors. Hypoglycemia (plasma glucose, 2.2 ± 0.3 nmol) induced by insulin (3 IU/kg, ip) caused a 3- to 5-fold increase in the plasma concentrations of ACTH and β-END. A negative exponential correlation was found between the plasma glucose concentration and the ACTH and β-END levels. Pretreatment of the animals with the HA synthesis inhibitor a- fluoromethylhistidine (1.0 yumol) intracerebroventricularly (icv) in a lateral ventricle, inhibited the ACTH and β-END responses to insulin/hypoglycemia by 60%. When administered ip (100 μmol/kg), the synthesis inhibitor decreased the β-END response 50%, but did not affect ACTH secretion significantly. Pretreatment of the rats with the H₃-receptor agonist R(α)methylhistamine (50 μmol/kg, ip, twice) inhibited the secretory responses of ACTH and β-END to insulin/hypoglycemia by 60-80%. This inhibitory effect of R(α)methylhistamine was reversed by prior administration of the specific H₃-receptor antagonist thioperamide. Administration of the H₁-antagonists mepyramine and cetirizine dose-dependently inhibited the ACTH and β-END responses to insulin/hypoglycemia, with the highest dose (mepyramine, 350 nmol, icv; cetirizine, 40 μmol/kg, ip) inhibiting the response by 80-100%. The H₁-antagonist SKF-93944 (226 nmol, icv) inhibited the ACTH response, but had no effect on the β-END response. Administration of the H₂-antagonists cimetidine (400 nmol, icv) and ranitidine (400 nmol, icv) inhibited the ACTH and β-END responses to insulin/hypoglycemia by 50-80%. We conclude that histaminergic neurons are involved in the mediation of the insulin/hypoglycemia-induced release of ACTH and β-END and that the effect is mediated via activation of primarily postsynaptic H-receptors and, to a lesser extent, H₂-receptors.