Prevention of low bone mass is important to reduce the incidence of osteoporotic fractures. In man, the consumption of fruits and vegetables is associated with greater bone mineral density (BMD), an effect that is claimed to be caused by their base excess buffering metabolic acid, thought to dissolve bone. We showed previously that in the rat the consumption of several vegetables, salads, and herbs inhibits bone resorption and that onion increases bone mass. In this study we show that, although the intake of onion is associated with a decrease in urinary noncarbonic acid excretion and a concomitant inhibition of bone resorption of similar magnitude, the two findings are not causally related. Onion retains its bone resorption inhibitory activity in the rat even when added to a vegetarian diet with typical base excess. Onion and a mixture of vegetables, salads, and herbs retain their inhibitory activity even when metabolic acid is buffered with potassium citrate. In addition, neither the pH nor the potassium content of individual ashed vegetables, salads, and herbs correlates with inhibition of bone resorption. The effect of vegetables, salads, and herbs, which inhibit bone resorption in the rat, therefore is not mediated by their base excess but possibly by a pharmacologically active compound(s).