Mitral regurgitation was created surgically in 10 dogs. After measurement of baseline hemodynamic parameters on day 2, treatment with captopril (2 mg/kg orally 3 times daily) was begun in five dogs. Five control dogs received no therapy during the study. The hemodynamic parameters were measured again at weeks 6 and 22. In the treated dogs, the forward ejection fraction (FEF) increased slightly and the total peripheral resistance index (TPRI) decreased. In the controls, the FEF decreased and the TPRI increased. Changes in FEF and TPRI were significantly different between the two groups. The change in FEF correlated significantly with the change in TPRI in both groups. Although the most dramatic changes were observed at week 6, the differences persisted throughout the study. Captopril administration resulted in sustained hemodynamic improvement and may, therefore, delay the onset of congestive heart failure in dogs with mitral regurgitation.