These studies examined the role of bulbospinal serotonin-containing neurons found in the nucleus raphe magnus and nucleus raphe pallidus in the mediation of morphine-induced antinociception. Lesions were made using both electrolytic coagulation and the axon-sparing technique of monosodium-L-glutamate injection to ascertain whether the effects following lesions in the area of the medullary raphe nuclei are due to destruction of neuronal perikarya or fibers passing near these nuclei. These studies revealed that lesions of both the raphe magnus and raphe pallidus resulted in decreased nociceptive thresholds and attenuation of morphine-induced analgesia. Such effects were observed regardless of the lesioning method used, which suggests that destruction of neurons in these nuclei was responsible for lesion-induced effects. In addition, lesion-induced changes in spinal cord serotonin content and morphine analgesia were significantly correlated which lends support to the conclusion that the bulbospinal serotonin systems are necessary for the mediation of morphine effects. Furthermore, no correlation was observed between changes in spinal cord norepinephrine content and morphine analgesia. This observation suggests that lesion-induced damage to bulbospinal noradrenergic fibers which pass near the midline does not contribute to the attenuation of morphine analgesia resulting from raphe lesions.