1. The rostral ventrolateral medulla (RVLM) is essential for the normal control of arterial pressure. This region of the brainstem is heterogeneous, and contains spinally projecting neurons that contain phenylethanolamine-N-methyltransferase (C1 neurons) and non-C1 neurons. 2. The availability of a toxin, a dopamine-beta-hydroxylase antibody conjugated to saporin, that can selectively destroy C1 neurons within the RVLM allows for the study of cardiovascular regulation in rats following depletion of the C1 neuronal population. 3. C1 neurons are not essential for the maintenance of resting blood pressure in anesthetized rats, but do contribute to many cardiovascular responses mediated through the RVLM. 4. The depressor response elicited by clonidine injected into the RVLM is dependent upon C1 neurons. 5. Studies in rats with selective toxin-induced destruction of RVLM C1 neurons demonstrate that C1 neurons contribute importantly to cardiovascular regulation.