Cardiovascular, respiratory and analgesic effects of fentanyl and naloxone were studied in normotensive acutely decerebrated dogs. Naloxone (1 mg/kg, i.v.) increased skin twitch reflex latency, mean blood pressure, pulse pressure, respiratory rate and minute volume. Fentanyl (50 micrograms/kg, i.v.) decreased heart rate and blood pressure while the animals were artificially ventilated. The skin twitch reflex latency was not significantly altered. Nine minutes later, naloxone (1 mg/kg, i.v.) was administered and the fentanyl-induced cardiovascular depression was reversed above the control level. The skin twitch reflex latency remained unchanged. These findings give further evidence that the endogenous opioid system plays an important role in the brainstem control of circulation and respiration. The mechanism of the anomalous analgesic response of the acutely decerebrated dog requires further investigation.