A special metabolic cage system was employed to measure the intestinal, renal, and mammary gland fluxes of Ca, P, and Mg in vitamin D-deficient rats during late pregnancy and lactation. Dietary Ca, P, and Mg levels were 0.78, 0.34, and 0.083%, respectively; this diet minimizes the reduction in milk production observed during vitamin D deficiency. Compared with identically treated virgin rats, lactating rats were slightly hypocalcemic and severely hypophosphatemic. Hypertrophy of the small intestine, as indicated by increased intestinal length and villus height, occurred during lactation. Net fractional intestinal absorption of Ca and P, but not Mg, was elevated twofold during late pregnancy and throughout lactation. Despite this elevated intestinal absorption, lactating rats were in negative Ca and P balance and lost bone mass. The transfer rates of Ca, P, and Mg into milk were approximately 77% of values previously observed in vitamin D-replete rats. Lactating rats conserved P by dramatically reducing renal P excretion. Pup retention of ingested Ca was virtually complete. These results, together with previous observations using everted duodenal gut sacs, indicate that there is a vitamin D-independent stimulation of intestinal Ca and P absorption during pregnancy and lactation. Because fractional Mg absorption was not similarly enhanced, this stimulation shows some specificity.