Five young, mildly obese females consumed a formula diet providing total calories at 1.2 X basal energy expenditure and egg white protein, 1.5 gm./kg. ideal body weight (IBW), for 1 week (period 1). During period 2, lasting 3 weeks, a protein-sparing modified fast (PSMF) consisted of similar amounts of egg white without nonprotein calories followed by a 1-week total fast (period 3). In the final period (4), a PSMF with the use of meat protein, 1.5 gm./kg. IBW, alone was given for 1 week. Nitrogen balance (Nbal) measurements were made in all periods, and periods 2, 3, and 4 were compared with period 1. No significant difference existed between period 1 and period 2, although Nbal improved weekly during period 2. Nitrogen excretion in period 3 was similar to that found after 3 weeks of total fasting and was significantly negative, whereas positive balances occurred in period 4 with both periods being significantly different from period 1. Thus nitrogen equilibrium in the PSMF can be achieved with the use of protein free from fat, confirming earlier experience with meat protein. The metabolic adaption which occurs during toal fasting and results in reduced nitrogen loss develops in the course of PSMF. After a deficit in lean body mass is produced, net protein anabolism can be achieved by a PSMF despite insufficinet dietary energy.