Studies have demonstrated abnormalities in baroreflex function in various models of hypertension. The purpose of this study was to examine baroreflex sensitivity in the canine model of obesity-induced hypertension. Baroreflex sensitivity was determined by the relative changes in heart rate (HR) in response to changes in mean arterial pressure (MAP) induced by sodium nitroprusside (Nitro) and phenylephrine (Pe). Studies were performed in conscious lean dogs (n = 10) and obese dogs fed a high-fat diet for 6 wk (n = 8). Body weight averaged 18.1 +/- 0.3 kg in the lean dogs and 26.5 +/- 0.5 kg in the obese dogs. Associated with the 46% increase in body weight in the obese dog group were significant increases in MAP (120.7 +/- 3.0 vs. 102.8 +/- 1.5 mmHg) and HR (132.7 +/- 8.5 vs. 96.4 +/- 3.3 beats/min). Administration of Nitro (0.5, 1.0, 5.0 micrograms.kg-1.min-1) resulted in comparable reductions in MAP in the lean and obese dogs. The reflex increases in HR were significantly greater in the obese animals only with a dose of 5.0 micrograms.kg-1.min-1 of Nitro (55.5 +/- 7.7 vs. 32.0 +/- 7.5 beats/min). Administration of Pe (0.5, 1.0, 5.0 micrograms.kg-1.min-1) resulted in significantly greater increases in MAP in the obese dogs (17.8 +/- 4.6 vs. 4.9 +/- 2.6, 37.1 +/- 4.4 vs. 19.6 +/- 2.7, and 72.7 +/- 7.5 vs. 51.5 +/- 7.1 mmHg).(ABSTRACT TRUNCATED AT 250 WORDS)