The effects of maternal vitamin D deprivation on rat pups were examined in terms of serum levels of Ca, phosphorus (P), and 25-hydroxyvitamin D (250HD), ash content, and histological appearance of long bones. In each of 10 experiments, litters of 8--10 young were examined at 2 or 3 different ages within the range of 5--57 days. Pregnant mothers were fed a diet containing 0.4% Ca, and 0.4% P and either no vitamin D(--D) or 5 IU D3/g diet (+D). Mothers fed the --D diet developed marked hypocalcemia (4--5 mg/dl) during lactation, but no change in milk Ca or P. The earliest changes in pups suckled by mothers on --D diets compared to pups suckled by mothers on +D diets were significant reductions at 8 days in serum P (6--12% in 3 of 5 experiments) and in serum 250HD3. By the 15th day, serum 250HD3 was undetectable, body weight was reduced by 26%, serum Ca was reduced by 9%, and serum P was reduced by 15%. During the following 10 days, when gradual weaning took place, serum Ca decreased progressively, serum P returned to normal, serum Mg became elevated, bone ash (as percent of dry weight of tibia) was decreased, and decalcified sections of the proximal end of the tibia revealed irregularity and widening (1.5- to 3-fold) of the hypertrophic cartilage layer and the metaphyseal trabeculae. All of these changes were also prominent in 8-week-old --D rats. The results indicate that rat pups suckling vitamin D-deprived mothers can develop biochemical evidence (including hypophosphatemia) as well as histological evidence of vitamin D deficiency similar to that of human vitamin D deficiency rickets. This is in contrast to rats deprived of vitamin D after weaning, who do not develop hypophosphatemia or characteristic histological evidence of rickets unless deprived of P as well as vitamin D.