Analysis of existing N balance data in adult man has shown that when body weight is constant protein requirements are regulated, much like energy. Data relating to daily N balance on fixed intakes have been examined for the nature and degree of intra-individual variation. It has been shown that for intakes in the range of 3.5 to 12 g N/day, the day-to-day fluctuations in N balance are not random but are serially correlated in an autoregression process. This implies that the daily N balance, like energy balance, is regulated. This regulation is produced by a probabilistic generating mechanism which remains constant through time. At very high or negligible N intakes this regulation is shown to break down, i.e., homeostasis can no longer be maintained. At high levels of protein, metabolism is altered, becoming more rapid than before, body weight increases, and the frequency and amplitude of oscillations become larger and irregular. At low levels of protein, body weight decreases, and the frequency and amplitude of the oscillations increase and decrease, respectively. In either case, the organism is under stress. The interpretation of the autoregression model is that the daily requirements for man in health will be distributed around a constant mean with stationary variance. It has been shown that the magnitude of this variance is comparable with the variation between individuals. The result is found to hold even when the daily requirement is averaged over several days. We conclude that protein deficiency must be defined as a failure of the process to be in statistical control, and not defined in the manner that assumes requirements to be fixed whereby if an individual consumes protein below this level, he suffers from protein deficiency. Based on the autoregression model, a method has been indicated for estimating the incidence of protein deficiency in the population.