Wild House mice, Mus musculus, bred at 23°C (controls) changed little in reproductive performance over ten generations. Similar mice bred at 3°C (Eskimo) became more fertile and heavier. Eskimo body fat also rose. Control adrenal weights declined; Eskimo adrenal weights were heavier than those of controls, but only during the first four generations. The Eskimo phenotype after ten generations was a combined result of a direct response to cold, parental effects and genetical changes in the Eskimo population. Maternal effects were probably especially important. In such conditions, the minimum unit of selection that it is useful to consider is the female and her litter.