Physostigmine (P) was used as a model in anesthetized rats for the development of hypertension by a central cholinergic mechanism. P (25-100 micrograms/kg i.v.) produced a dose-related increase in mean arterial pressure which was inhibited 30 to 90% by preinjection of clonidine (100 micrograms/kg i.v.). This dose of clonidine was without effect on the pressor response to ganglionic stimulation produced by 1,1-dimethyl-4-phenylpipera-zinium iodine. To examine further the central inhibitory effect of clonidine on the pressor response to P, regional brain acetylcholine turnover rate was measured in control and clonidine-pretreated rats. Clonidine significantly reduced turnover in hypothalamus (69%), pons-medulla (43%) and midbrain (39%) but not in striatum or hippocampus. These observations are consistent with an inhibitory effect of clonidine on central cholinergic neurons involved in cardiovascular regulation.