1. Weanling rats, aged 21 d, given a 40 g casein/kg diet for 25 d were compared with ‘age-matched’ controls given a 200 g casein/kg diet and with ‘weight-matched’ weanling controls.

2. The protein-malnourished rats demonstrated failure of weight gain and linear growth, hypoproteinaemia, hypoalbuminaemia and fatty liver.

3. Autoradiography of the tibia was carried out 24 h after intraperitoneal injection of Na235SO4 (0.1, 1.0, 2.0, 5, 10 μCi/g body-weight). Sections of tibia, humerus and costochondral junction were stained by haematoxylin and eosin, resorcin-crystal violet and picro-fuchsin, Alcian Blue at pH 0.1 and pH 2.5, Alcian Blue at pH 5.7 with magnesium chloride (0.1, 0.7, 1.0 m), and by Alcian Blue after ovine testicular hyaluronidase (EC 3.2.1.35) digestion. The width of the upper tibial epiphysis was measured and the histological and histochemical features of the epiphyses studied.

4. The incorporation of 35SO4 into the epiphyses of the protein-malnourished animals was markedly reduced. The chondrocytes were small and flattened with frequent pyknotic nuclei. The staining characteristics of the cartilagenous matrix was qualitatively similar in all animals. The epiphyses in malnutrition were found to be thin, all zones being affected. The upper tibial epiphyses (mean±sd; μm) were 202 ±46 for the protein-malnourished animals, 367±52 for age-matched controls and 578±40 for weight-matched controls.

5. The changes found resemble those after hypophysectomy and the possible hormonal factors are discussed.