1. The hypotensive effect of green beans, common rue and kelp was recently shown in normotensive rats in vivo. A number of mechanisms of action of these aqueous extracts was identified. The present study examined these actions at the tissue level in vitro with possible interactions of these extracts. 2. Rue showed positive chronotropic and inotropic effects on isolated right atria. Green beans and kelp alone showed negative chronotropic effects on isolated right atria but no effect on atrial tension (AT). A combination of green beans and kelp showed no additive effect on the decrease in atrial rate (AR) nor any negative inotropic responses. Combinations of rue and green beans and of rue and kelp showed responses that were either positive or negative chronotropically, were not dose dependent and were less than the sum total of their individual responses (i.e., subtractive). A combination of all three showed subtractive effects on the decrease in AR that were dose related. No change in AT was observed upon treatment with a combination of the three plant extracts in spite of the positive inotropic effect of true. 3. Rue and kelp alone relaxed KCl preconstricted rat tail artery strips probably by a direct effect of vascular smooth muscle. Green beans had no effect. The combination of rue and kelp exerted a subtractive relaxation effect. These plants therefore contained cardiovascular active substances that had a direct effect on the cardiovascular system. These substances further interacted to modify their cardiovascular effects. 4. Data explained why herbs, as in herbal medicine, should be used together therapeutically.