The binding of [3H]hemicholinium ([3H]HCh-3) to sodium-dependent high-affinity choline uptake sites provides a useful neuroanatomical and functional marker of the cholinergic system. We examined the autoradiographic distribution of [3H]HCh-3 binding sites in the forebrain of young (4-6 months) and old (32 months) rats. There was a widespread reduction of [3H]HCh-3 binding site density in the aged rat brain. This loss presented regional differences with maximal reduction in the medial and posterior striatum (55%) and in the dentate gyrus (47%), in limbic areas such as basolateral amygdala, tubercle olfactorium and piriform cortex the autoradiographic signal was about 25-30% lower. In aged hippocampus and cerebral cortex the density of [3H]HCh-3 binding sites was about 40% lower, the difference between young and senescent animals being less evident in the medial septum and basal nucleus. No significant alterations were observed in interpeduncular nucleus from old rats. These data are in agreement with the functional results obtained by measuring other cholinergic parameters in the aged rat and confirm the vulnerability of cholinergic system during aging.