Free fatty acid (FFA) concentrations were determined in arterial plasma samples obtained during short or prolonged fasts from resting, conscious adult male mongrel dogs. Arterial blood was continuously collected during experiments ranging from 2.25 to 15 h, from dogs fasted 1 or 5 days. Each continuous collection stream was divided at 45 equal intervals into separate sequential samples. In 18 experiments, no consistent pattern was found in plasma FFA concentrations: the concentrations were never constant, and fluctuations were only rarely periodic. Large, spontaneous, seemingly random level changes with amplitudes of 200-600 mueq/liter were observed in 14 experiments, that took from 1 to 8 h to complete. There was no apparent relation between these fluctuations and time of day or rectal temperature, and glucose concentrations remained unchanged during the FFA fluctuations. The inconstancy of plasma FFA levels and the irregularity of their fluctuations suggests that FFA production rates may be unregulated or only loosely regulated within a wide regulation band during the first few days of starvation. No current model of FFA metabolism preducts the observed patterns of FFA levels.