Four groups of day-old turkey poults were fed a synthetic vitamin D-deficient diet. Poults received weekly, progressively increased oral repletion of: corn oil, group I; vitamin D3, group II; 1,25(OH)2D3, group III; 1,25(OH)2D3 plus 24,25(OH)2D3, group IV. Poults were examined for clinical and structural changes of bone and parathyroid glands;Poults in group I rapidly developed ricketic changes by day 10. In the growth plates of the proximal tibiotarsus, the lengths of the proliferating-prehypertrophied and unmineralized hypertrophic chondrocyte zones increased rapidly in conjunction with a reciprocal decrease in length of the mineralized hypertrophic degenerative zones. This reciprocal exchange occurred faster in the proximal tibiotarsus than in the distal tibiotarsus. Other significant findings included: decreases in bone ash, bone ash (calcium, phosphorus and magnesium), bone length, and body weight. Bone-ash, Ca/P ratios and plasma alkaline phosphatase actively increased. There were normocalcemia, hyper-to-normophosphatemia, parathyroid hyperplasia, and rapid depletion of vitamin D3 metabolites. Ultrastructurally, hyperplastic parathyroid chief cells had expanded cytoplasmic volumes, reduced numbers of secretory granules, and tortous, interdigitated plasma membranes. Cartilage matrix vesicles (80-113 nm) and bone matrix-like vesicles (53-133 nm) were seen in all four groups;Group II poults (control) did not develop rickets. Group II poults had low plasma levels of 25OHD3 and 1,25(OH)2D3 indicating that growing turkey poults may require higher levels of vitamin D3 than NRC requires;Groups III and IV had significant mild ricketic changes: elongated proliferating-prehypertrophied zone with irregular distal margin and elevated plasma alkaline phosphatase. Total bone ash calcium and phosphorus levels of group IV were similar to group II but group III values were significantly less than group II. These results suggest that 24,25(OH)2D3 synergistically restores bone levels of calcium and phosphorus; thus, it is an important vitamin D3 metabolite. These results also suggest that vitamin D3 is more effective than 1,25(OH)2D3 and that 24,25(OH)2D3 provides no synergism in preventing all ricketic changes.