The present study was designed to assess the effect of supplementation with dietary cytidine (5')-diphosphocholine (CDP-choline), a source of cytidine and choline, on memory in young and older rats. Although the hippocampal-dependent memory deficits in aged rats are well documented, cognitive functioning in early aging has not been as thoroughly evaluated. Female Sprague-Dawley rats (3 or 15 months of age) consumed either a control diet or a diet supplemented with CDP-choline (approximately 500 mg/kg/day) for 8 weeks, after which they were trained to perform spatial and cued versions of the Morris water maze. Compared with young rats, aged rats exhibited a selective deficit in spatial memory tasks that required rats to retain information for 24 h or longer. CDP-choline supplementation protected against the development of this deficit, but had no memory-enhancing effect in normal young rats. These findings suggest that early-aged rats display a selective impairment in hippocampal-dependent long-term memory, and that dietary CDP-choline supplementation can protect against this deficit.