Study of data in the literature on rat and mouse brain growth from birth to weaning reveals a stagewise growth in average brain weight. Rapid growth occurs in the intervals between days 0–6, 8–12, and 17–23 after birth. Slow growth periods then lie in the intervals 6–8, 12–17, and after 23 days. The first slow growth period is signalled by events occurring at its end: substantial acceleration of synthesis of RNA, DNA, protein, and myelin. The second slow growth period is characterized by at least a 3-day interval during which there is very little increase in average brain weight compared with what occurs just before and just after that period; the correlation among 12 studies is highly significant. Implications are discussed for cross-species’ extrapolation of findings about brain development.