Forty-nine neurons recorded within the nucleus raphe dorsalis (NRD) in awake cats were classified into 2 groups: 29 regularly firing (clock-like) and 20 irregularly firing (non-clock-like) neurons. Hardly any of the clock-like neurons were influenced either by noxious stimulation (0.1 ml of 5% formalin, s.c.) or by a single dose (1 mg/kg, i.p.) or cumulative doses (0.25, 0.5, 1 mg/kg) of morphine. In contrast, about half the non-clock-like neurons were activated both by noxious stimulation and by administration of morphine. Morphine-induced activation of non-clock-like neurons was dose-related and reversed by naloxone (0.2 mg/kg, i.p.). These findings suggest that clock-like neurons in the NRD are not involved in morphine analgesia. Non-clock-like neurons, however, may play a role in the mediation of such analgesia.