The hemodynamic and hormonal responses to acute and chronic captopril therapy and to its temporary withdrawal were studied in seven patients with congestive heart failure. Maximal hemodynamic and hormonal effects were reached with 25 to 50 mg doses of captopril. Since plasma angiotensin II levels were significantly higher 6 1/2 hours than 1 hour after administration of captopril, the drug should be given not less often than three times daily. No evidence of hormonal "escape" during long-term (mean 4 1/2 months) captopril therapy was observed, and initial hemodynamic responses were well maintained. Cessation of captopril administration resulted in abrupt increases in circulating angiotensin II levels, in arterial pressure, and in both pulse rate and plasma norepinephrine, but no decrease in cardiac function in the short-term was detected.