Sources of variation in broiler experiments involving Ca, P, the vitamin D derivative 1-hydroxycholecalciferol, and phytase were investigated in four separate studies. Study one investigated the variation in the responses of broiler chickens due to exogenous phytase supplementation at different levels of dietary Ca and P. Phytase was found to be most efficacious when added to diets deficient in P but adequate or high in Ca. Reducing the level of Ca or increasing the level of P reduced the efficacy of the enzyme. Study two was conducted to examine the Ca requirements of broiler chickens during the starter (0 to 16 d) and grower (19 to 42 d) periods of development. The Ca requirements for starter chicks were similar to what is recommended by the National Research Council (1994) for maximizing bone mineralization (1.0%). The requirements for grower chickens appeared to be significantly lower than what is recommended (0.9%). The objective of Study three was to evaluate the implications of replacing a significant portion of inorganic Ca and P with 1-hydroxycholecalciferol and phytase in diets of fast growing broiler chickens raised under conditions which closely resemble those found in commercial broiler houses in the United States. Two similar experiments were conducted where bird performance and bone quality were measured at the end of both the starter (0 to 18 d) and grower/finisher phases (19 to 35 d) and the chickens were processed and their carcasses dissected to assess carcass quality at the end of each experiment. Birds in these two, apparently identical, experiments responded quite differently to the same diet formulations emphasizing the importance of experiment to experiment variation in experiments involving Ca, P, vitamin D metabolites and phytase. Study four evaluated the significance of the level of vitamin D3 fed to broiler breeder hens on variation in performance and bone disease of the progeny when fed a tibial dyschondroplasia inducing diet. Results showed that the level of maternal vitamin D3, hens age, egg production and particularly egg size influence body weight gain and the severity and incidence of tibial dyschondroplasia of the progeny.