Changes in autonomic function were studied during experimentally induced weight changes in seven subjects. A spectral analysis of heart rate variability (HRV) was used to evaluate autonomic activity during weight change. With a 10% increase in body weight above the usual or starting weight, there was a decline in parasympathetic power accompanied by a rise in mean heart rate. Heart rate declined during weight reduction, but the power of HRV did not change significantly. Because heart rate power at the frequency of respiratory rate can be affected by respiratory rate, three additional subjects were tested at a constant respiratory rate. Weight increases in this group also led to a decline in the power of HRV at a frequency attributable to the parasympathetic nervous system. Such a parasympathetic effect of weight increase may be one mechanism for the arrhythmias and other cardiac alterations that accompany obesity.