The distribution in the rat brain of high affinity nicotinic heteromeric acetylcholine receptors and of low affinity nicotinic, alpha7-containing, homomeric receptors was studied using in vitro light microscopic autoradiography. As ligands, we used [3H]epibatidine, or [125I]epibatidine, and [125I]alpha-bungarotoxin, respectively. In adult animals, the two types of binding sites were widely distributed in many different brain structures, including the brainstem, cerebellum, mesencephalic structures, limbic system and cortex, but their anatomical distribution differed markedly. Only in rare instances could a co-localization be observed, for example in the superficial layer of the superior colliculus. In developing animals, both types of labeling were strongly expressed during embryonic and postnatal phases. Their distributions were qualitatively similar to those observed in adult animals, with a few noticeable exceptions in the cerebral cortex, hippocampus and brain stem. In aging animals, neither the distribution nor the density of nicotinic binding sites was significantly altered. Our conclusions are the following. (a) There is little overlap in the distribution of heteromeric and alpha7-containing homomeric nicotinic receptors in the rat brain. (b) The abundance of neuronal nicotinic receptors during embryonic and postnatal development suggests that they may play a role in the establishment of neuronal connectivity. (c) The expression of neuronal nicotinic receptors is unaltered in middle aged animals, suggesting that in the rat these receptors do not play any major role in aging process.