Dairy cows suffer from an intense energy deficit at parturition due to the onset of copious milk synthesis and depressed appetite. Despite this deficit, maternal metabolism is almost completely devoted to the support of mammary metabolism. Evidence from rodents suggests that, during periods of nutritional insufficiency, a reduction in plasma leptin serves to co-ordinate energy metabolism. As an initial step to determine if leptin plays this role in periparturient dairy cows, changes in the plasma concentration of leptin were measured during the period from 35 days before to 56 days after parturition. The plasma concentration of leptin was reduced by approximately 50% after parturition and remained depressed during lactation despite a gradual improvement in energy balance; corresponding changes occurred in the abundance of leptin mRNA in white adipose tissue. To determine whether negative energy balance caused this reduction in circulating leptin, cows were either milked or not milked after parturition. Absence of milk removal eliminated the energy deficit of early lactation, and doubled the plasma concentration of leptin. The plasma concentration of leptin was positively correlated with plasma concentrations of insulin and glucose, and negatively correlated with plasma concentrations of growth hormone and non-esterified fatty acids. In conclusion, the energy deficit of periparturient cows causes a sustained reduction in plasma leptin. This reduction could benefit early lactating dairy cows by promoting a faster increase in feed intake and by diverting energy from non-vital functions such as reproduction.