In spontaneously hypertensive rats (SHR) a diminished responsiveness for nociceptive stimuli is present, which might be associated with alterations in endogenous opioid peptide and receptor systems. In SHR, normotensive Wistar-Kyoto controls (WKY) and in Wistar rats the effects of acute morphine administration on pain sensitivity and body temperature have been investigated. Morphine potency decreases significantly in the rank order Wistar, SHR, WKY. Concerning body temperature, a low dose (2.5 mg/kg i.p) results in an initial and transient decrease in body temperature in SHR, which is not elicited in Wistar rats and WKY. After higher doses (10 and 20 mg/kg. i.p) in Wistar rats profound hypothermia occurs whilst SHR and WKY show a modest increase in body temperature. These differential effects in the three strains could not be explained by altered morphine concentrations in subcortical brain tissue.