To evaluate factors that might be related to the pathogenesis of hypertension in spontaneously hypertensive rats (SHR), mineral balance studies were conducted in SHR and control Wistar-Kyoto (WKY) rats at the ages of 9, 13 and 16 wk. During the first balance period, before onset of hypertension, Na, Ca, Mg and P balances were all significantly more positive in SHR than in WKY rats. Percentage of absorbed Na excreted in urine was correlated with the rise in blood pressure of SHR but not of WKY rats during the first 4 wk of the study. At 13 wk balances of all four nutrients still tended to be higher in SHR. During the 16th wk of life, after onset of hypertension, Na, Ca and P balances were similar in SHR and WKY rats, but Mg balance was significantly lower in SHR. The percent urinary excretion of absorbed Na was lower in SHR at 9 wk of age, but after onset of hypertension it was similar to that in WKY rats. Food intake was always greater in SHR, whereas growth during the study was less, and absorption of Ca, Mg and P declined more rapidly with age in SHR than in WKY rats. The data suggest that nutrient malabsorption in SHR, if it occurs at all, is most likely an effect of, rather than a cause of, hypertension.