Weanling Thoroughbred horses were fed diets providing 70%, 100%, or 130% of their daily energy and protein requirements for eight months. Biopsy specimens of growth plate cartilage were taken from the distal right radius at this time. Tissues from both overfed and underfed horses exhibited significantly decreased protein, hydroxyproline and hexosamine contents (on a dry tissue weight basis), increased DNA content and decreased LDH activity, compared to tissues taken from the animals fed 100% of their daily requirements. Growth plate thickness was proportional to diet level. The reserve and hypertrophic zones were enlarged in the overfed horses. The hypertrophic cartilage from the overfed horses lost the usual columnar organization of lacunae and remained unpenetrated by metaphyseal capillaries. Longitudinal growth rates of the upper forelegs were reduced by under-nutrition and were unaffected by overfeeding. The similarities between the lesions accompanying chronic overfeeding and those of equine hypothyroidism suggest endocrine involvement linking dietary excess to developmental osteochondrosis.