A study was made relating the anticholinergic action of alpha 2-adrenoceptor agonist clonidine on the cardiovascular responses to i.c.v. injection of neostigmine, a quaternary anticholinesterase, in conscious rats. The cholinergic agent neostigmine (0.1-1 microgram i.c.v.) induced a dose-dependent pressor and bradycardic responses in the rats. Clonidine (10 micrograms.kg-1 i.v.), given 30 min previous the anticholinesterase, failed to modify the pressor effect of neostigmine (0.1-1 microgram i.c.v.) but prevented the bradycardia. On the other side, the i.c.v. administration of clonidine (3 micrograms), given 15 min before neostigmine, only prevented the bradycardic effect of neostigmine (0.3 microgram i.c.v.) but not the central pressor action of this drug. The results support the idea that clonidine has an anticholinergic action in the rat. However, prevention by clonidine of central cholinergic bradycardia suggests different cholinergic-adrenergic interaction on the cardiovascular responses elicited by central administration of cholinergic agents.