SummaryThe effects of prolonged administration of excess dietary dl-methionine and niacinamide on growth rate and on the relative weights of the liver, kidneys and adrenal glands of rats were determined. Although both methionine and niacinamide caused a decrease in the growth rate, a greater retardation of growth was obtained when both nutrients were administered together. The addition of 1% malic acid to the diet prevented the inhibition of growth by these substances. Although methionine caused a transient enlargement of the kidneys, none of the treatments appeared to place a severe stress on the animals.