This paper is one of a series presenting right-left differences in the morphology of the rat forebrain, but this presentation differs from the previous ones by offering age-related changes in both sexes. Long-Evans rats were housed with the dam prior to weaning at 21 days of age and three to a cage thereafter. The ages of the animals studied were 6 to 7, 14, 21, 90, 180 to 185, 390 to 400, and 870 to 876 days. The thicknesses of the cerebral cortex and of the hippocampus were measured on microslide-projected images of thionin-stained sections. We learned that the cerebral cortex of the male rat was thicker on the right side than on the left at all ages in 41 of 42 measures, being statistically significant in 30 of 42 measures. Areas 10, 3 and 17 showed the most marked differences at all ages. In the female rat, laterality was not so well defined, but, in general, the left cerebral cortex was thicker than the right in 33 of 54 measures, but in only 5 of the 54 were statistically significant differences found. The right-left differences in the hippocampus followed the pattern of the cortical differences in the male and female rats. The right male hippocampus was thicker than the left at all ages, with greater differences noted in the younger than in the older groups. The female left hippocampus was thicker than the right, but only in the 90-day group was the difference significant.