It has been proposed that the performance of lactating animals is limited by the capacity of the female to dissipate body heat – the heat dissipation limit (HDL) theory (Król and Speakman 2003, Speakman and Król 2010). This theory predicts that milk production might be constrained not by intrinsic properties of the mammary glands but rather by competitive heat production such as thermogenesis in brown adipose tissue (BAT). To test this prediction, we measured the expression of genes linked to thermogenesis in BAT of lactating laboratory mice. The applicability of BAT gene expression to reflect thermogenic activity of BAT was confirmed by a positive relationship between expression levels of several BAT genes (summarised by the first principal component following principal component analysis) and daily energy expenditure in virgin mice. Milk production at peak lactation was strongly and negatively associated with the expression of thermogenic genes in BAT (Fig. 1). Downregulation of these genes during lactation was correlated with low levels of circulating leptin and high levels of circulating prolactin (Fig. 2). We used path analysis to further examine associations between hormone levels, BAT activity (reflected by the PC1 scores) and milk production in lactating mice. Low levels of circulating leptin were correlated with and may contribute to the increase in milk production by reducing BAT activity and by BAT-independent mechanisms. High levels of circulating prolactin were correlated with increased milk production via associations with the activity of BAT rather than by BAT-independent mechanisms, but the relationships with prolactin were weaker than those with leptin. Our results are consistent with the prediction of the HDL theory that downregulation of BAT thermogenesis reduces competitive heat production, permitting greater production of lactogenic heat within the overall heat dissipation capacity limit. However, we cannot discount an alternative explanation that the causality in the associations is the reverse, i.e. that greater milk production, directly via thermogenic mechanisms alone (Cannon and Nedergaard 2004) or via endocrine and physiological processes, reduces BAT activity. The highly coordinated downregulation of thermogenic genes in BAT during lactation provides a new model system to investigate molecular pathways involved in switching on and off BAT activity and function.