In primates, the principal nociceptive pathways ascend in the anterolateral quadrant of the spinal cord. Among these, the spinothalamic tract (s.t.t.) is the best studied. Cells in Rexed's laminae I and V project to the ventro-posterolateral (v.p.l.) thalamic nucleus. Other cells in the same and deeper laminae terminate in the intralaminar complex. Spinothalamic tract cells may be nociceptive-specific or multireceptive. Those ending in v.p.l. have restricted, contralateral receptive fields, whereas those projecting to the intralaminar region often have large, bilateral receptive fields. Spinoreticular tract (s.r.t.) cells are concentrated in laminae VII and VIII and may be nociceptive. It is proposed that the s.t.t. contributes to sensory-discriminative processing of pain and that the s.t.t. and s.r.t. play a role in the motivational-affective components of pain. Alternative nociceptive pathways are the spinocervical and postsynaptic dorsal column tracts.