Abstract: Three experiments examined the effects of nutrition on the development of the tibia in broiler chicks. One experiment used 0.05% increments of nonphytate phosphorus (NPP) from 0.25 to 0.50% of the diet. Chicks fed 0.35% NPP or less had lower plasma P and lower bone ash than those fed 0.40% or more NPP. Growth plates from P deficient chicks were enlarged. Microscopic examination of the growth plates revealed blood vessel penetration of the hypertrophic zone but failure to remove the chondrocytes. The Ca experiment used 0.1% increments from 0.6 to 1.1% of the diet. These treatments did not affect plasma Ca or bone ash. Chicks fed 1.1 or 1.0% Ca had a low incidence of enlarged growth plates. Diets with 0.9% Ca or less caused more than half of the chicks to have growth plates longer than 2 mm. Most of the enlarged growth plates were classified as tibial dyschondroplasia (TD), but some from the 0.6 and 0.7% Ca diets were classified as rickets. In both defects, metaphyseal blood vessels failed to penetrate the growth plate. The accumulated cells in TD had progressed through hypertrophy but had regressed to the size and shape of prehypertrophic chondrocytes. In rickets, the accumulated cells were proliferative. The basal vitamin D diet had 165 ICU/kg^-1 . Vitamin D was increased in five other diets by multiples of 1.81. Dietary vitamin D content had no effect on bone ash or on plasma Ca or P content. If chicks were fed 992 ICU/kg^-1 or less, the proportion of chicks with enlarged growth plates was increased. The defect was classified as TD and appeared to be identical to that resulting from a Ca deficiency. The development of the blood vessels in the growth plate seems to be the most useful criterion for deciding if an enlarged growth plate is due to a Ca or P deficiency.