Intravascular volumes and systemic and regional hemodynamic variables were measured before and after weight reduction in 12 patients with obesity and essential hypertension. These findings were compared with those in nine patients who did not have any weight loss. Reduction of mean arterial pressure significantly correlated with the fall in total body weight (r = 0.46, p less than 0.05). Total circulating and cardiopulmonary blood volumes were significantly reduced (p less than 0.05 and p less than 0.01, respectively), and these changes permitted a decreased venous return and cardiac output (p less than 0.01). This fall in cardiac output was directly related to a contracted total blood volume (r = 0.49, p less than 0.05) and decreased cardiopulmonary blood volume (r = 0.52, p less than 0.05). Patients who did not lose weight showed no changes in any of these hemodynamic measurements. In addition, weight loss was associated with reduced resting circulating levels of plasma norepinephrine (p less than 0.01), suggesting that diminished adrenergic function may also be related to weight reduction and its associated fall in arterial pressure.