The effect of acute administration of morphine on cerebral excitability was investigated in rats with two convulsant drugs: flurothyl (hexafluorodiethyl ether) and pentylenetetrazol (PTZ). In the flurothyl study, adult male Sprague-Dawley (S-D) rats were injected subcutaneously with morphine sulfate in doses ranging from 0.5 to 256 mg/kg. At 15, 30, 60 and 120 minutes after morphine injection, flurothyl was administered by inhalation and the seizure thresholds were determined. In the PTZ study, 64 mg/kg of morphine sulfate were injected subcutaneously into both S-D and CFN (Wistar-derived) rats. Thresholds to PTZ seizures were measured after administering the convulsant either by the intraperitoneal or intravenous route. The data revealed an anticonvulsant action of morphine on both flurothyl and PTZ. Peak time for this effect on flurothyl seizures was 30 minutes after subcutaneous administration of the opiate, with the maximal anticonvulsant activity appearing at the 64-mg/kg dose. The increase in seizure threshold in S-D rats at this dose was 36% with flurothyl, 94% with intravenous PTZ and 352% with i.p. PTZ. Morphine had a less dramatic influence on raising the latter seizure threshold in the CFN than in the S-D strain. The graded dose-related anticonvulsant action is independent of the respiratory depression associated with morphine administration and appears to be a reflection of an altered central nervous system excitability produced by the narcotic in rats.