In rats and cats anesthetized with alpha-chloralose and urethane, the pressor response and its relative reactivity were studied following microinjection of dl-homocysteic acid (DLH, 2 nmol in 40 nl of saline) into the dorsomedial medulla (DM) or the ventrolateral medulla (VLM). DLH, which excites only cell bodies of neurons but not fibers of passage, consistently produced pressor responses in DM and VLM in both cats and rats. The pressor effects elicited from VLM were more pronounced than those from DM. In cats, the most active areas for DLH were found in the rostral and midmedulla; the pons and caudal medulla were less active. The pressor responses from DM and VLM were augmented following bilateral vagotomy, persisted after sectioning of the carotid sinus and aortic depressor nerves, and after cauterization of the carotid bodies with phenol. The response induced by DLH was more apparent from VLM than that from DM. These pressor effects were evoked directly by activation of neurons in these two regions, and were not necessarily related to any homocysteate blockade of baroreceptor and/or chemoreceptor reflexes. These results suggest that in the medulla there reside at least two discrete pressor areas, DM and VLM containing neuronal perikarya.