Three different experiments were carried out on growing male Sprague-Dawley rats. In the first experiment, dietary nitrogen was given in casein at four different protein values ranging from 5 to 16%. Rats were fed ad libitum for 21 days. In the second experiment, which lasted 17 days, animals were given three diets that differed in the molecular form of the nitrogen supply - that is, proteins or peptides. The protein value (N x 6.25) was 10% in each diet. In the third experiment, malnourished rats were refed diets with a protein value of 15% (N x 6.25) for eight days. The dietary nitrogen was either in the form of protein, peptide, or amino acid. Body weight was recorded daily. At the end of each experiment the intestinal villus height was measured by light microscopy. Data were statistically analysed by Exner's coefficient. The results assessed the validity of the correlation between villus height and gain in body weight.