The rates of gain, catabolism, synthesis and reutilization of myofibrillar protein were measured in adult rats fed a protein-free diet, low protein diet (2% lactalbumin) or control diet (10% lactalbumin) for 14 to 31 days. Two forms of synthesis were measured: exogenous synthesis (nitrogen derived from diet) and endogenous synthesis (nitrogen derived from catabolized body protein). The rate of gain of myofibrillar protein was measured as the rate of increase in its weight and the rate of catabolism was determined from urinary 3-methylhistidine excretion. The rate of total synthesis was calculated as the sum of these two rates. Exogenous synthesis was calculated from the recovery of isotope in protein 24 h after oral administration of 15N-leucine and endogenous synthesis was calculated as the difference between the total synthesis and exogenous synthesis. Reutilization was calculated as the ratio of endogeneous synthesis to catabolism. The rate of catabolism was slightly decreased in protein deficiency (2.1, 2.1 and 2.6% in the protein-free, low protein and control groups, respectively), while that of synthesis was significantly decreased in protein deficiency (1.3, 2.0 and 3.3% in the respective groups). Restriction of protein intake resulted in a decrease in the rate of exogenous synthesis, without appreciable change of endogenous synthesis. The reutilization rate of endogenous N was estimated to be about 70% in rats with restricted protein intakes and about 50% in those with a normal protein intake.