A total of 30 cats were used in this experiment.The nucleus raphe magnus wasinjected with 0.1～0.3?l of 25～50% HRP solution(RZ:2.4～3.1).Two or three daysafter operation,the animals were killed and then perfused with the fixing fluid.Thebrainstem and spinal cord were sectioned serially on a freezing microtome and treatedaccording to the method of retrograde axonal transport of horseradish peroxidase.Parts of the sections were counterstained with toloidine-blue.The labelled neuronswere examined microscopically under bright-field and dark-field illumination Thelabelled neurons were distributed as follows.After injections of nucleus raphe magnus,a few labelled neurons were foundin the medial part of parafascicular nucleus of the thalamus.A greater numbers oflabelled cells were consistently found in the ventrolateral area of the central greymatter of the midbrain.The dorsal nucleus of the midbrain raphe had a few labelled neurons.The HRPpositive neurons were found in the nucleus Darkschewitsch,mesencephalic reticularformation,nucleus reticularis gigantocellularis,nucleus reticularis paramedianus,nucleus vestibularis medialis and nucleus vestibularis lateralis.In some cases a fewlabelled cells were also seen in the posterior area of hypothalamus,nucleus posteriorcommissure,nucleus tegmentalis dorsalis,superior collicus,nucleus annularis,reticularformation of the pons,nucleus nervi facialis and nucleus gracilis.In summary,the present studies show that the afferent connections of nucleusraphe magnus are widely concerned with the structures of the pain control.Itsuggests that the nucleus raphe magnus is an important relay station for pain control.