The origins of brain stem projections to the cytoarchitectonically different areas 17, 18 and 19 of the cat's visual cortex were studied following small horseradish peroxidase (HRP) injections. Labelled cells were counted in a dopaminergic nucleus (nucleus linearis rostralis (NLR)), other catecholaminergic nuclei (locus coeruleus, parabrachialis nuclei and nucleus subcoeruleus) and serotonergic nuclei (nucleus raphe dorsalis (NRD) and nucleus centralis superior (NCS)). Area 18 receives afferents from more locus coeruleus cells than either of areas 17 or 19. The number of labelled cells in the catecholaminergic nuclei far exceeds that in the serotonergic nuclei.