The effect of ritodrine hydrochloride on hypoxic pulmonary vasoconstriction (HPV), the normal control mechanism for shunting blood flow away from nonventilated areas of the lung, was studied in nonpregnant dogs equipped with central monitors and electromagnetic flow probes. The systemic infusion of ritodrine at a dose of 4 micrograms/kg/min resulted in a 66.4% +/- 4.6% decrease in the HPV response, whether administered before or after the induction of isolated lobar hypoxia. These findings have significant implications for the patient who develops pulmonary edema during ritodrine therapy, in which inability to bypass nonventilated areas of the lung would serve to aggravate further the ventilation/perfusion inequalities that already exist.