Leptin is involved in the regulation of food intake, energy expenditure and whole body energy balance. In livestock species these processes are important for optimisation of growth, reproduction, lactation and overall health and well-being. Condition score is a useful indicator of energy balance and is highly correlated (-0.84) genetically to commencement of luteal activity (CLA), an endocrine measurement of fertility in dairy cattle (Royal et al., submitted). Circulating leptin levels drop post-partum reflecting changes in energy balance and fat mobilisation. The interval to first ovulation post-partum is phenotypically correlated (0.83) to the interval from parturition to the leptin nadir (Kadokawa et al. 2000), suggesting that a delay in the recovery of leptin secretion postpartum increases the delay to first ovulation. The heritability of circulating leptin concentrations in humans and pigs is high (0.39-0.61; Cameron et al., 2000; Rotimi et al., 1997). If this were also the case in cattle, and postpartum changes were related to concentrations at an early age, then it would be of immense value to the genetic improvement of traits associated with fertility.