Several neonatal and adult parameters were determined in the Roderick-Wimer lines of mice selected for low (L) or for high (Hb and Hg) adult brain weight, and in the unselected control line (C). Neonatal cerebral weight, DNA (index of cell number), and protein were significantly higher in the H and C lines than in the L line. The ratios of cerebral DNA/weight (index of cell density), cerebral protein/weight, and cerebral protein/DNA (index of protein content per cell or cell size) were at birth practically the same in all lines. In the adult animals all the differences were amplified and, in addition, some of the differences in the above ratios were significant, for the cerebral cortex as well as for the cerebellum. These findings may be causally related to the reported superiority in behavioral performance of the H line.