Mothers of young children are a population sub-group with one of the lowest levels of physical activity. This paper presents the findings from a qualitative study with 40 Australian mothers of children under school age. The research aimed to understand the tensions, dilemmas and trade-offs which women experience around physical activity within the contexts of their everyday lives as mothers of young children. The analysis shows that, in contrast to health promotion messages which frame physical activity as a positive and healthy behaviour, mothers of young children perceive activity as being both enhancing and threatening to their health and social relationships. Restrictive stereotypes of the 'good' mother make it difficult for many women to prioritise their own physical activity needs over their childrearing and domestic responsibilities. Nevertheless, women's involvement in physical activity is often underpinned by the maternal 'ethic of care' as something which can help them cope better with the challenges of being a mother and contribute to the wellbeing of the family. This article takes as its departure point the notion that the maternal 'ethic of care' creates previously unrecognised opportunities in relation to physical activity. For many mothers, physical activity can also be a way of challenging hegemonic discourses and extending what it means to be a good mother in contemporary society. Although largely overlooked by contemporary health promotion, it is women's family-oriented politicism and resistance to dominant meanings about motherhood, health and the 'ideal' body which create alternative possibilities for their participation and enjoyment of physical activity during early motherhood.