The incidence of osteoporotic fractures has been associated with low bone mass. To reduce this incidence, it is therefore important to try to prevent the development of low bone mass by either increasing bone mass built up during adolescence and/or preventing bone loss in later life. It has been shown that food fractionation, a procedure that prevents the diurnal rhythm of bone resorption, increases bone mass in growing rats fed a high calcium (Ca), high phosphate (Pi) diet. In this paper, data are presented that show that providing growing rats with the same daily amount of a high Ca, low Pi diet (a Pi content similar to that of a human diet) in portions every 6 h instead of one meal increases total bone mineral content, trabecular bone mineral density, and cortical thickness, and markedly reduces the decrease in these parameters in aged rats. The effect is smaller when a high Ca, high Pi diet is fractionated. This could be the consequence of the transient postprandial increase in parathyroid hormone (PTH) following one large meal of a high Ca, high Pi diet, since the effect is similar to that reported after PTH injections which are anabolic for bone. Thus, a high dietary Pi must be considered as a confounding factor when treatments affecting bone mass are investigated in rats. The present data show that feeding habits have a profound effect on bone mass in the rat, independent of age. Whether bone mass in humans is also under the control of dietary habits is not yet clear. If so, frequent small meals of appropriate composition may help to prevent osteoporosis.