This study determined whether opiates alter vascular components of inflammation (hyperthermia, edema and plasma extravasation) in addition to the suppression of hyperalgesia. Rats were administered carrageenan into one hind paw and saline into the other hind paw, followed by i.p. injection of morphine (0.2-5.0 mg/kg) or saline at 60 min, and testing at 90 min after hind paw injections. Morphine produced a dose-dependent reduction in carrageenan-induced hyperalgesia (17-53%), hyperthermia (39-53%) and edema (24-36%). Morphine treatment did not alter the temperatures of the contralateral saline-injected paws, indicating that opiate suppression of hyperthermia was not confounded by alterations in systemic body temperature or blood flow. The opiate effects on inflammation were stereospecific since levorphanol (1 mg/kg), but not dextrorphan (1 mg/kg), suppressed carrageenan-evoked hyperalgesia, hyperthermia and edema. Pre-treatment with naltrexone (1.5 mg/kg) blocked the effects of a 5 mg/kg dose of morphine sulfate on hyperalgesia, hyperthermia and edema. In a separate study, i.v. injection of morphine sulfate (2 mg/kg) reduced plasma extravasation by 41% (P less than 0.01). Morphine administration resulted in significantly greater increases in paw withdrawal latencies in the inflamed (38-139%) than the contralateral, saline-treated paws (4-19%). The results indicate that opiates exert a moderate, though significant, reduction in the vascular signs of inflammation in addition to their reduction of hyperalgesia. The mechanisms for this vascular effect involve inhibition of both vasodilation (as indicated by a decrease in hyperthermia) and inhibition of vascular permeability. In addition, opiates exhibit enhanced antinociceptive effects in inflamed paws, even when compared to uninjured paws in the same animal.