Effects of morphine were studied in 36 gallamine triethiodide immobilized adult cats under light N2O anesthesia. Single units were recorded from the nucleus dorsal raphe using a stainless steel microelectrode. Dorsal raphe neurons were divided into two types; one was a clock-like (CL) neuron which was typically slow in rate, rhythmic and stable throughout recording time, and the other was a non-clock like (NCL) neuron which was relatively irregular in pattern as compared with CL neuron. Out of 36 neurons recorded in this experiment, 13 were CL neurons and 23 were NCL neurons. Mean discharge rates (spikes per sec) of CL neuron and NCL neuron were 2.84 (range of 2.01 approximately 3.68) and 4.11 (range of 0.10 approximately 35.09), respectively. None of the 13 CL neurons responded to the nociceptive (pinch and/or brdaykinin) and non-nociceptive (hair bending and/or tapping) stimuli. On the contrary, out of 23 NCL neurons, 13 responded to both nociceptive and non-nociceptive stimuli, 4 were responsive to only non-nociceptive stimuli and 6 were not responsive to these stimuli. Receptive fields of NCL neurons were wide with various somatic modalities. The latency for bradykinin (3 microgram) was 6.46 +/- 0.77 sec. Effects of morphine were examined in 6 CL neurons and 12 NCL neurons. After morphine, no nociceptive neuron responded to nociceptive stimuli, although there was no appreciable change in the responsiveness to non-nociceptive stimuli. Firing frequency and pattern of all CL neurons were unaffected by morphine. Out of 12 NCL neurons, 7 were unaffected in firing frequency and 5 were decreased after morphine.