The effects of dietary restriction (calorie and/or protein) imposed at weaning on tissue concentrations of nucleic acids and protein were determined in young (one year) and old (two year) rats. In liver, calorie restriction increased protein concentration and decreased RNA/DNA and RNA/protein, while protein restriction elevated liver DNA and reduced protein and protein/DNA. These changes are consistent with decreased liver cell size in response to protein, but not calorie, restriction. Age also influenced these parameters, since DNA (elevated), RNA/protein (elevated), protein (reduced), and protein/DNA (reduced) differed in livers of old as compared to young rats. In kidney, DNA concentrations were unaffected by age or dietary restriction. Although protein and nucleic acid concentrations are commonly used as baselines for expressing other biochemical parameters, our results indicate that in liver, these factors vary both with age and nutritional status.