We estimated the effects of dietary administration of docosahexaenoic acid (DHA, 22:6n-3) on rat spatial cognition using young male Wistar rats. Chronic administration of DHA to rats increased the hippocampal levels of DHA. Six weeks after the start of the administration, the rats were for 5 weeks trained to acquire a reward at the end of each of 4 arms of an 8-arm radial maze. The retention test interposed with a delayed task (20-min interval after the first 2 free choices) was then given to each rat to determine spatial working memory retention. On completion of the test, the Fos expression in the hippocampus was examined immunohistochemically. DHA administration reduced the number of working memory errors after the 20-min interval in the retention test. The number of Fos-positive neurons in the CA1 hippocampus demonstrated a statistically significant negative correlation with the number of working memory errors, suggesting that the DHA-induced improvement in spatial cognition is associated with increased Fos expression in the CA1 hippocampus.