The effects of lysergic acid diethylamide (LSD), methysergide, and cyproheptadine on activity in classical primary pathways of the visual and somatosensory systems were compared with their effects on activity in sensory convergent (association) regions in alpha-chloralose-anesthetized cats. Those effects were blocked by cyproheptadine whereas methysergide potentiated the actions of LSD on visual primary activity. In contrast, LSD depressed the primary somatic pathway, at small doses (25 to 50 micrograms/kg) and facilitated the response at larger doses (200 micrograms/kg). Cyproheptadine and methysergide did not agonize these actions of LSD. The anterior marginal cortex, nucleus central median-parafascicularis, nucleus lateral posterior, and the superior colliculus, all sites of heterosensory convergence, were depressed by LSD. The depression of responses at heterosensory sites by LSD was blocked by cyproheptadine. Methysergide potentiated the LSD-induced depression of visual-evoked activity but not somatosensory activity. These results suggest that LSD depresses sensory activity in regions which integrate multiple sensory modalities independently of actions on sensory-specific pathways. These effects appear to involve a cyproheptadine-sensitive system.