In order to determine whether age-related changes in neuronal membrane structure contribute to previously reported changes in muscarinic cholinergic signal transduction, striata from 3, 13 and 23 month old F344 male rats were examined for both carbachol-stimulated low Km GTPase activity and membrane one-dimensional electron density profile using small angle X-ray diffraction. Increasing age was associated with both a reduction in stimulated GTPase activity and a decrease in membrane bilayer width. These findings suggest the possibility that fundamental membrane structural changes may contribute to alterations in signal transduction seen with aging.