Male and female weanling Sprague-Dawley rats were fed either an AIN-76A diet or a modification of the AIN-76A diet containing no added DL-methionine but with higher levels of vitamins, fluoride and magnesium than in the AIN-76A diet. Both diets were fed, to groups of ten rats of each sex, at 18% protein or a reduced protein level of 13% for 12 wk. Within each sex, all diets produced comparable weight gains in rats at the end of 12 wk, except that the reduced-protein modified AIN-76A diet was associated with a reduction in weight gain in male rats. Both diet and protein level had statistically significant effects on the relative weights of some organs, particularly the kidney. The AIN-76A and the reduced-protein AIN-76A diets significantly increased the relative kidney weights (% body weights) of female rats, when compared with the effects of both modified AIN-76A diets (18 and 13% protein). Male rats fed both of the diets containing 18% protein had higher relative kidney weights than did those consuming both 13% protein diets. Females fed the modified diet containing 13% protein had significantly lower liver weights than the other groups. In both sexes, the two diets containing 18% protein caused significantly higher plasma urea nitrogen concentrations than did the lower protein diets. Kidney calcium concentrations varied with the diet, with dietary protein level, and with the sex of the animal. All diets caused small mineral (calcific) concretions of minimal to mild severity in the lumina of scattered renal tubules in the cortex and/or medulla of male rats. All female rats fed the AIN-76A and the reduced-protein AIN-76A diet had large, moderate or severe mineral concretions in the tubules at the corticomedullary junction and this was associated with increased renal calcium levels. The higher concentration of renal calcium at the lower dietary protein level (13%) was associated with severe corticomedullary junction mineralization. The higher protein diets were associated with an increased incidence of hyaline droplets in the cytoplasm of kidney cortical tubules in male rats.