The growth pattern of the brains of normal CD-1 mice was studied from day 1 through day 21 after birth. The main purpose of the study was to decide on suitable methods of statistical analysis of these kinds of data including whether to analyze brain weight or the brain-to-body weight ratio. A secondary purpose, which aided the main purpose, was to find functions that describe the relationships between growth parameters. Linear-linear segmented polynomials were found to describe well the relationships between (a) brain weight and body weight, (b) brain weight and age, (c) brain-to-body weight ratio and body weight and (d) brain-to-body weight ratio and age. The analyses indicated that brain weight alone is the preferred variable to analyze. Regression analysis of brain weight with sex and up to the first three powers of body weight and age as independent variables would be a suitable method of analysis. Litter size per se had no effect on brain weight.