Studies demonstrating an importance effect of dietary intake on the activity of the sympathetic nervous system of the rat are described. Utilizing techniques to measure the turnover rate of norepinephrine in heart, fasting has been shown to suppress, while overfeeding sucrose stimulates, the sympathetic nervous system. Studies in the spontaneously hypertensive rat (SHR) demonstrate that diminished caloric intake, or fasting, with maintenance of normal sodium intake, lowers the blood pressure; conversely, overfeeding sucrose on a constant sodium intake increases blood pressure in the SHR. Overfeeding an isocaloric ration of fat, in contrast to sucrose, has no effect on blood pressure in the SHR. Blood pressure responses in the normotensive Wistar-Kyoto (WKY) strain are in the same direction but much less marked than in SHR. The sympathetic nervous system response to fasting and sucrose overfeeding in the SHR resemble those that occur in WKY and other normotensive rat strains, at least in so far as the heart is concerned. These studies, thus, raised the possibility that diet-induced changes in sympathetic nervous system activity may contribute to diet-induced changes in blood pressure. According to this hypothesis the beneficial effect of weight loss in obese hypertensive subjects, and conversely, the detrimental effect of weight gain on the development of hypertension may reflect corresponding diet-induced changes in sympathetic activity. The well known relationship between obesity and hypertension may, therefore, derive at least in part from an effect of dietary intake on the sympathetic nervous system.