The level of sympathetic nervous system (SNS) activity in obesity is controversial, with reports claiming either increased or decreased SNS activity. The following studies examined SNS activity in a dietary form of obesity, ingestion of a lard-enriched diet for 4 wk. Plasma norepinephrine (NE) levels were 61% higher in rats fed the lard-enriched diet than in chow-fed controls at 20 degrees C (200 +/- 24 pg/ml vs. 124 +/- 6, P < 0.005) and remained elevated after 1 h of cold exposure (4 degrees C). [3H]NE turnover was markedly increased in heart, but not in interscapular brown adipose tissue (IBAT), kidney, liver, skeletal muscle, or spleen of rats fed the high-fat diet. By contrast, ingestion of a diet similarly enriched with sucrose raised rates of [3H]NE turnover in IBAT as well as in heart. Thus chronic ingestion of a lard-enriched diet induces region-specific stimulation of SNS activity that is greater in heart than in IBAT. Whereas the absence of an SNS response to lard in IBAT may contribute to weight gain in these animals, activation of cardiac sympathetic nerves may promote development of hypertension in this model of obesity.