Abstract The morphology of cerebral cortical laterality patterns differs between the sexes. In the male Long-Evans rat, the thickness of the cerebral cortex is, in general, greater on the right side than on the left, with many areas showing statistically significant differences. In the female Long-Evans rat, the left side is thicker more often than the right, but the differences, in general, are not statistically significant. These laterality patterns are maintained throughout the lifetime of the animal with few variations. Some of the male and female laterality patterns reverse with old age. The numbers of neurons and glial cells in the area sampled, area 39, support the direction of cortical thickness measurements in male and female rats. Removal of the testes or ovaries at birth alters the usual cortical laterality patterns, illustrating that the sex steroid hormones play some role in determining laterality. In the neonates of both sexes, estrogen receptors are found in the cerebral cortex, but the concentration is greater in the left male cortex than in the right, the opposite being true for the female. Factors other than the sex steroid hormones, such as stress, can alter cortical laterality. Many studies indicate that plasticity of laterality is a factor to be considered when dealing with cortical morphology and, in turn, behavior.