Systemically administered nicotine elicits ACTH release indirectly by acting on neurons in brainstem catecholaminergic regions known to send afferent projections to the paraventricular nucleus of the hypothalamus (PVN), the site of CRH neurons involved in initiating ACTH secretion. The present study in rats examined 1) the relationship between dose-dependent expression of cFos in the PVN and that in the nucleus of the solitary tract (NTS)-A2, NTS-C2 and locus coeruleus (LC), after iv nicotine (0.045-0.18 mg/kg, administered at 0.09 mg/kg per min); 2) the dependence of PVN cFos expression on the effects of nicotine in brainstem, using the nicotinic cholinergic antagonist, mecamylamine, administered into the fourth ventricle; and 3) the extent of catecholaminergic involvement in the effect of nicotine on the PVN, measured by immunocytochemical double-labeling for cFos and tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. The results showed that the magnitude of cFos expression was dependent on the dose of nicotine in all regions studied (P < 0.0006); however, at the two lowest doses, only the NTS and CRH-containing region of the PVN expressed cFos, whereas the LC and the rest of the PVN were activated only by higher doses. Nicotine also elicited a dose-dependent increase in cFos expression in the TH+ neurons of the NTS, with C2 more sensitive than A2. Interestingly, the majority of NTS neurons expressing cFos were noncatecholaminergic, implicating other transmitter systems. Fourth ventricular mecamylamine completely blocked nicotine-induced cFos expression throughout the NTS, as well as the PVN. The results provide further support for the idea that catecholaminergic afferents from the NTS, but not the LC, play a significant, albeit not an exclusive, role in the activation of the PVN in response to nicotine.