Central excitatory potency of morphine administered by cerebroventricular infusion in enhanced in derivatives substituted at the 3-position (phenolic group) and/or 6-position (alcoholic group). Morphine-3-glucuronide is several hundred times more potent than morphine in evoking dose-related hyperactive motor behavior which can progress to lethal convulsions. Excitatory potencies in decreasing order are: (1) 3-glucuronide; (2) 3-SO4; (3) 3-OAc, 6-OAc (heroin); (4) 6-OAc; (5) 3-OAc; (6) 3-OH, 6-OH (morphine); (7) 3-OCH3 (codeine); (8) 3-OCH3, 6-OCH3 (thebaine). Levorphanol, lacking a 6-OH group, is devoid of excitatory actions. In this series of substituted morphines, there is an inverse relationship between opiate receptor binding potency and central excitatory potency, but codeine and thebaine behave anomalously. These findings are compatible with the hypothesis that morphine acts upon a species of receptor which mediates behavioral and EEG excitation and is distinct from the recognized opiate receptor mediating sedation and analgesia.