We investigated the effects of captopril (CAP) and L-arginine (ARG) on hypertension and cardiopulmonary function. Our hypothesis was that CAP therapy or ARG will improve cardiopulmonary risk factors for hypertension and hypoventilation in the obese Spontaneously Hypertensive Heart Failure rat that is characterized by hypertension, obesity, and disorders of lipid and carbohydrate metabolism. For the first study, one group of rats received CAP in drinking water and a second group received deionized water (DI). For the second study rats were further subdivided. Some CAP-treated rats continued on this treatment and the other half were now given DI to determine if there would be residual effects of CAP treatment. A subgroup of rats who had received DI was then given ARG while the rest remained on DI. In the first study CAP-treated rats exhibited decreases in systolic and diastolic blood pressures, frequency of breathing, minute ventilation, and maintained ventilatory control. In contrast, blood pressures and relative ventilation to metabolism were higher in the DI-treated group. Removal of CAP increased blood pressure and decreased tidal volume while these rats maintained frequency. Although ARG-treated rats did not exhibit a decrease of blood pressure, ventilation was maintained in this group by preserving tidal volume. Thus, CAP and ARG affected ventilation through different mechanisms independent of blood pressure.