This study examined the cardiovascular, respiratory, and sympathetic effects of selective mu and delta opioid agonists microinjected into the hypothalamic nucleus preopticus medialis (POM) of conscious SHR and WKY rats. The mu receptor agonist D-Ala2-MePhe4-Gly5-ol-enkephalin (DAGO) at a dose of 0.6 or 6.0 nanomoles (Nmol) increased the blood pressure and heart rate in WKY rats. In SHR rats, the lower dose of DAGO similarly had a pressor effect whereas the higher dose was depressor; heart rat was increased only by the 6.0 nmol dose in these animals. In both SHR and WKY rats, this opioid caused respiratory acidosis and elevation of plasma norepinephrine (NE) and epinephrine (E); plasma vasopressin was reduced by the higher dose of DAGO. All of these effects of the mu agonist were reversed by the opiate receptor antagonist naloxone (0.5 mg/kg, i.a.). The delta opiate-receptor agonist D-Ala2-D-leu5-eukephalin at a dose of 6.0 or 20.0 nmol increased blood pressure and heart rate in both SHR and WKY rats without affecting respiratory variables. Plasma NE and EPI were elevated at the peak of the pressor period. These studies suggest that the anteroventral hypothalamic region may be an important site in central autonomic regulation by opioid peptides. The mu-receptor agonist was more potent than the delta agonist in eliciting cardiovascular and respiratory effects and associated sympatho-adrenomedullary activation.(ABSTRACT TRUNCATED AT 250 WORDS)