The effects of maternal vitamin D3 carry-over to the poult and dietary vitamin D3 on kidney 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) (EC 1.14.13.13) activity were studied in poults from 6 days prehatching to 46 days of age. Large White female turkeys from day-old were fed diets with either 300, 900, or 2700 IU vitamin D3/kg feed. Progeny from each maternal group were fed diets with either 0, 300, 900, or 2700 IU vitamin D3/kg feed. Low maternal vitamin D3 carry-over increased kidney 1-hydroxylase activity in embryos and poults to at least 5 days of age. Low dietary vitamin D3 increased the enzyme activity over that of poults fed higher levels. Kidney 1-hydroxylase activity peaked at about 8 to 18 days to a level similar in all groups. This peak is coincident with the appearance of rickets often noted in the field. Maternal diet did not affect plasma calcium (Ca) but plasma inorganic phosphorus (Pi) decreased with decreasing maternal vitamin D3 up to 14 days of age. Plasma Ca increased at 14 days with higher dietary vitamin D3, as did plasma Pi from day 8. Plasma alkaline phosphatase increased with age to 18 days and then declined. Tibia ash increased with higher maternal vitamin D3 carry-over to 12 days and with higher dietary vitamin D3 after 12 days of age. Body weight was reduced with low maternal vitamin D3 carry-over until at least 2 weeks of age, after which the effect of progeny diet was highly significant; birds receiving 2700 IU vitamin D3/kg feed were almost twice as large as those receiving none. This study shows the importance of adequate maternal carry-over of vitamin D3 and its possible influence on the development of rickets in starting poults.