The aging process has long been associated with a decrease in neuronal cell numbers in several brain areas. Recent applications of stereology have challenged this hypothesis by suggesting that the aging process is characterised by neuronal dysfunction, and perhaps axonal degeneration, rather than neuronal death. In this thesis, stereology was used to establish the status of cholinergic septohippocampal neuron number, size and fibre length in aged compared to young rats. We found that the number and the cross-sectional area of choline acetyltransferase (ChAT)-positive cell bodies remained constant in aging. However, the length of hippocampal ChAT-positive fibres was significantly reduced in aging (32% decrease). No changes in the activity of ChAT and the levels of nerve growth factor and brain-derived neurotrophic factor were detected in the hippocampus or medial septum. In conclusion, new stereological probes developed to measure fibre length densities represent a sensitive and powerful tool to evaluate axonal degeneration.