The effects of vitamin D deficiency on fertility, reproductive capacity and on fetal and neonatal development were investigated. Female weanling rats were maintained on either a vitamin D-replete or vitamin D-deficient diet until maturity and mated with normal males. Vitamin D-deficient females were capable of reproduction. However, vitamin D deficiency reduced overall fertility by 75%, diminished litter sizes by 30% and impaired neonatal growth from day 6 to day 15 of lactation. Fetal development asjudged by weight gain and viability appeared normal. Neonatal viability was also normal even though growth was retarded. The concentrations of calcium and inorganic phosphate in milk from vitamin D-replete and vitamin D-deficient mothers were similar implying that the transfer of calcium and phosphorus from the plasma to the milk is a vitamin D-independent process.