Adjustments to sympathetic nervous system activity may regulate constant body weight despite wide variations in energy intake. To test this six normal weight subjects were studied at three different energy intakes (low, weight maintaining and high). Noradrenaline turnover was measured on the tenth day of each diet. Both noradrenaline appearance rate and noradrenaline clearance increased significantly with increasing energy intake and were a more sensitive indices than the plasma noradrenaline concentrations which rose, but not significantly. Fasting triiodothyronine (T3) rose and reverse T3 fell with increasing energy intake, while thyroxine (T4) concentrations did not change. Systolic blood pressure also rose significantly. Underfeeding resulted in reductions in noradrenaline appearance and clearance rates and in the T3 level. These results demonstrate that sympathetic nervous system activity, as determined by noradrenaline turnover in plasma, varies in response to short-term changes in energy intake in normal weight subjects. These changes may partly explain why some individuals maintain body weight constant despite large differences in food intake. The present findings may also be relevant to the variability in susceptibility to become obese.