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THE crossfostering technique was used to investigate the dynamics of genetic and maternal effects on growth in a random bred population of ICR mice. In addition to weights taken every 3 days from birth to 6 weeks and weekly weights thereafter until 84 days, several gains, 6-week tail length and traits derived from the logistic and Richards growth functions were considered. For weights, the variance due to genetic dams was initially large, but from day 6 to day 49 varied from about 10 to 15% of the phenotypic variance. After day 49 this component became relatively more important. The variance due to nurse dams peaked at 12 days accounting for 68% of the phentoypic variance in 12-day weights. After 12 days a progressive diminution of the relative importance of this component was found. A theoretical consideration of the crossfostering design indicated that the direct-maternal genetic covariance was equal to the interaction component of variance between direct genetic and nursing effects of dams, in the absence of fostering effects. This component was generally small and positive for weights and gains. For tail length and gains, the nurse component was more important than the direct genetic component with the exception of 42 to 84-day gain. Genetic variability and postnatal maternal effects on growth function traits were found. The growth function traits considered appear to be low to moderately heritable. It was concluded that of two functions considered, the logistic function was more suitable. Both functions underestimated asymptotic weight.