In a variety of mammals, the spatial ability is better in males than in females, and the activity in the novel environment is less in males than in females, and these sex differences have been believed to be genetically determined. In the present study, we examined whether the sensory stimuli brought about by chewing diet affected the spatial ability and the activity in the novel environment. We confirmed that, in rats fed a standard laboratory hard diet, male rats performed better than female rats in the radial arm maze task and exhibited less activity than female rats in the open field task, as previously shown. We however found that, in rats fed a soft diet, sex difference in the radial arm maze task was not found, in association with increases in the expression of NMDA receptor subunit R1 mRNA in the hippocampus and in the amount of acetylcholine release in the prefrontal cortex in female rats. There was no change in the sex difference in the open field task. We conclude that constitution of diet during development affects some aspects of cognitive functions, but not emotional functions. One is better to pay attention what environmental conditions are good for both sex. [Jpn J Physiol 54 Suppl:S242 (2004)]