Midbrain stimulation in chronically implanted, awake cats in the vicinity of dorsal raphe nucleus evoked profound analgesia to peripherally applied noxious stimuli. Analgesia was not accompanied by other obvious sensory or motor deficits. The peripheral field of analgesia sometimes included the entire body and in other cases was restricted to the hindquarters. Analgesia generally outlasted brain stimulation by no more than 10 sec. Stimulation distant from dorsal raphe led to pronounced motor or emotional effects precluding an adequate test of analgesia. In anesthetized cats, effects of midbrain stimulation were studied on evoked activity in dorsal horn lamina 4 and 5 cells. Stimulation of wide regions of midbrain inhibited pain-evoked activity in most lamina 5 cells without affecting responses of most lamina 4 cells. In some lamina 5 cells, midbrain stimulation inhibited responses to noxious but not innocuous stimuli. The possible role of lamina 5 cells in nociception and the possible relationship between analgesia and lamina 5 cell inhibition are discussed.