Northern elephant seals, Mirounga angustirostris, fast completely from food and water during lactation. Previous investigations of maternal investment suggested physiological constraints on the level of energy expenditure during lactation. In this study, two components of phocid fasting physiology, protein sparing and reduced glomerular filtration rate, were examined for effects of changing body composition and lactation duration. Protein catabolism was estimated from 14C-urea turnover in five mid- and five late-lactation females. Body composition was determined by using an ultrasound scanner to measure blubber depth coupled with morphometrics. Glomerular filtration rate was measured in five females at mid- and late-lactation using plasma clearance of 3H-inulin. Protein catabolism increased significantly between measurements. The contribution of protein to metabolism varied with body composition and lactation duration. Mass-proportional glomerular filtration rate increased significantly between measurements. These data suggest that conflicting metabolic demands of lactation and fasting might constrain the duration and magnitude of maternal investment in northern elephant seals.