Previous studies from our laboratory have demonstrated that dietary intake affects the sympathetic nervous system (SNS); carbohydrate intake, in particular, has been shown to stimulate sympathetic activity. The present studies were undertaken to characterize the effect of dietary fat on SNS activity in the rat. Sympathetic activity was assessed by measurement of norepinephrine (NE) turnover in heart, interscapular brown adipose tissue (IBAT), and pancreas and by excretion of NE in the urine. When fed a fat-enriched diet (50% chow, 50% lard), fractional NE turnover in heart (k) increased from 6.3 ± 0.6% h in ad lib. fed controls to 14.7 ± 1.3% h in the high-fat group (P < 0.001); calculated NE turnover rate increased rate increased from 24.5 ± 2.4 ng/heart per h to 36.8 ± 3.5 (P < 0.05). Urinary NE excretion more than doubled after 6 d of the same high fat diet (P < 0.001). Ganglionic blockade produced a greater effect on NE turnover in fat-fed, as compared with chow-fed animals, consistent with increased sympathetic activity in the fat-fed group. When fat absorption was blocked with a bile acid binding resin (cholestyramine), the same high-fat diet did not increase cardiac NE turnover, indicating that fat absorption is required for the stimulatory effect on sympathetic activity. In another series of experiments, in which chow (and hence protein) intake was held constant, the effect of fat and isocaloric sucrose supplements on NE turnover was assessed in heart, IBAT, and pancreas. The caloric value of the supplements was 50, 100, and 335% of the chow in the different experiments. An effect of fat on NE turnover in heart and IBAT was demonstrable at the lowest level of fat supplement. Fat increased pancreatic NE turnover when added in amounts sufficient to double the caloric intake. The stimulatory effect of sucrose and fat on NE turnover in heart and IBAT was similar. These experiments demonstrate that fat increases SNS activity in the rat and that the magnitude of the effect is similar to that of sucrose. The results imply that fat may contribute to dietary thermogenesis in the species.