We studied the hemodynamic response to orally administered hydralazine (30 mg) and examined the changes in neurohumoral factors in 16 aged patients with chronic congestive heart failure. After a single 30 mg dose, 9 patients (Group I) demonstrated a greater than 30% decrease in systemic vascular resistance (SVR) and a significant increase in the cardiac index (CI); 7 patients, whose SVR was decreased by less than 30% (Group II), showed a less marked increase in CI. After 3 days of drug administration (90 mg/day), the CI decreased in Group I (p less than 0.05) while it tended to increase further in Group II. Pretreatment plasma norepinephrine was lower in Group I than Group II (p less than 0.05). However, in Group I, it increased after 3 days of therapy, with a concomitant increase in plasma renin activity (p less than 0.01) and total plasma volume (P less than 0.025). We conclude that the higher pretreatment plasma norepinephrine level in Group II, suggestive of more severe derangement of left ventricular function, may be associated with a less pronounced dilatory response of the peripheral vasculature after a single dose of hydralazine, and that the increase in norepinephrine in Group I after 3 days of treatment may limit these patients' late hemodynamic responses to the drug.