Aged (26-month-old) rats were impaired compared with young (three-month-old) rats in passive avoidance and water maze tasks. In order to study whether changes in inhibitory circuits are involved in these age-related cognitive impairments, the number of two different subpopulations of GABAergic neurons, i.e. somatostatin- and parvalbumin-containing neurons, were counted in the hippocampal formation, septum and neocortex. We found that the number of parvalbumin-containing neurons was decreased in the entorhinal, somatosensory and motor cortex as well as in the medial septum and vertical limb of the diagonal band of Broca, but not in the hippocampus of aged rats. Somatostatin-containing neurons were affected in the somatosensory and motor cortex, and in the dorsolateral septum, but not in the hippocampus or in the entorhinal cortex. The decreased number of parvalbumin-containing neurons in the entorhinal cortex of the aged rats correlated with their performance deficits in passive avoidance and spatial learning. We propose that impaired functioning of the entorhinal cortex parvalbumin-containing inhibitory neurons may, to some extent, be responsible for the learning and memory defects found in aged rats.