The acetylcholinesterase inhibitor neostigmine (2 microg) was microinjected into the lateral cerebral ventricle (i.c.v.) of unanesthetized rats to activate central cholinergic receptors. Changes in arterial blood pressure were correlated with changes in Fos-like immunoreactivity in the hypothalamus and forebrain following cholinergic stimulation. Neostigmine increased mean arterial pressure by 39 +/- 3 mmHg at peak (P < 0.05) from a pretreatment level of 104 +/- 4 mmHg. Blood pressure remained elevated for more than 30 min. Distinct Fos-like immunoreactivity was found in the posterior hypothalamic nucleus, the paraventricular nucleus and the supraoptic nucleus of the hypothalamus, the ventral premamillary nucleus, the central nucleus of amygdala, the lateral septum and the medial preoptic area. In contrast, only a very small amount of Fos-like immunoreactivity was scattered in those regions in a control group injected i.c.v. with saline. Pretreatment with the muscarinic receptor antagonist methylatropine (i.c.v., 0.5 microg) prevented the pressor response to neostigmine and evoked a reduced Fos-like immunoreactivity compared to animals given neostigmine without methylatropine. The pressor response to neostigmine was blocked after pretreatment with phenoxybenzamine, however, this did not prevent the development of Fos-like immunoreactivity. These results indicate that the pressor response induced by central cholinergic stimulation may result from muscarinic receptor activation in specific regions of the hypothalamus and the forebrain that are implicated in regulating cardiovascular activity.