The diameter of cortical neuronal nuclei has been reported to increase when animals are reared in enriched, as opposed to deprived, sensory environments. The present study investigated whether analogous neuronal changes occur in the hippocampus. Male Wistar rats were reared in either an enriched or a deprived environment for 80 days after weaning. At the completion of this rearing period both brain weights and hippocampal neuronal nuclear diameters were measured. The enriched subjects exhibited a greater forebrain:hindbrain ratio (3.7% p less than 0.05) when compared to the isolated subjects. No significant differences in mean hippocampal nuclear dimensions could be detected but variability in nuclear size was greater in the stratum granulosum of the isolated subjects (p less than 0.01). It is proposed that this increased variability reflects an environmentally restricted development of these neurons.