1. Three battery experiments were conducted with broiler chicks during the 2nd and 3rd week of life. Graded amounts of cholecalciferol (D3) were added to maize-soyabean meal diets that were designed to be (a) severely deficient in available phosphorus (P), (b) marginally deficient in calcium (Ca) or (c) adequate in both available P and Ca. 2. With diets containing 1.0 g available P and 6.3 g Ca/kg (assay 1), graded doses of D3 between 0 and 37.5 mu/kg produced linear (P < 0.05) positive responses in both weight gain and tibia ash. With a D3 concentration of 1250 micrograms/kg, 250 times the requirement recommended by the NRC, bone ash was increased (P < 0.05) over that of birds fed 37.5 micrograms/kg, and neither weight gain nor food intake were reduced. 3. With a P-adequate diet (4.5 g available P/kg) containing 8.5 g Ca/kg (assay 2), weight gain and bone ash increased linearly (P < 0.05) upon supplementing the basal diet with 0, 2.5 and 5.0 micrograms D3/kg. Higher doses of D3 did not elicit further responses, and chicks fed on a diet containing 1250 micrograms D3/kg gained as fast and had bone ash values that did not differ from those of chicks receiving 5, 10, 20 or 40 micrograms D3/kg. 4. When the maize-soyabean meal basal diet was fortified with Ca and P to achieve adequate amounts of Ca (10.1 g/kg) and P (4.5 g available P/kg) in assay 3, dietary additions produced results similar to those obtained in assay 2 where P was adequate and Ca was slightly deficient. Again, chicks receiving a surfeit of D3 (1250 micrograms/kg) exhibited weight gains and bone ash values that were as great as those of chicks receiving 5, 10, 15 or 30 micrograms D3/kg. 5. It is apparent that young chicks have a high tolerance for excess D3, and chicks fed on diets that are severely deficient in available P continue to respond to D3 in excess of 37.5 micrograms/kg.