The purpose of this study was to determine whether genetically obese Wistar fatty rats have higher blood pressure than their lean littermates and if so to elucidate the mechanism of this obesity-related hypertension. We measured blood glucose and plasma insulin levels, blood pressure, and catecholamine and sodium excretions in age-matched female Wistar fatty and lean rats. After 12 weeks of age, the body weight of Wistar fatty rats was significantly greater than that of their lean counterparts. Fasting blood glucose and plasma insulin concentrations were higher in the fatty than the lean rats throughout the observation period (8 to 24 weeks of age). Systolic blood pressure of fatty rats measured by the tail-cuff method was similar to that of lean rats at 8 weeks of age (135 plus/minus 2 [mean plus/minus SEM] versus 134 plus/minus 3 mm Hg) but significantly higher at 16 (158 plus/minus 2 versus 136 plus/minus 3 mm Hg, P < .01) and 24 (166 plus/minus 5 versus 142 plus/minus 2 mm Hg, P < .01) weeks of age. Urinary norepinephrine excretion was significantly increased in the fatty rats at both 16 (1755 plus/minus 173 versus 977 plus/minus 128 ng/24 h, P < .05) and 24 (1907 plus/minus 283 versus 737 plus/minus 173 ng/24 h, P < .01) weeks of age. The ratio of urinary norepinephrine excretion to body weight was also significantly increased in the fatty rats. These results show that with increasing body weight Wistar fatty rats develop hypertension, which may be attributable to an increased sympathetic nerve activity. Since these characteristics are similar to those observed in human obesity-related hypertension, we consider that this rat may be a good model for further analysis of the mechanism of obesity-related hypertension. (Hypertension. 1995;25:146-150.)