All previous studies of Ca absorption in the developing animal have used simple salts of Ca. The aims of the current study are 1) to determine the predominant form of Ca in rat milk and 2) to compare the absorption of milk Ca and CaCl2. In vivo-labeled rat milk was obtained by injecting lactating dams with 45CaCl2. Distribution of 45Ca in the cream, whey, and casein fractions was determined by differential centrifugation, the values being 0.3, 5, and 95%, respectively. To study Ca absorption, rats aged 14 and 28 days received either 45Ca-milk or 45CaCl2 by intragastric intubation. At 14 days, transport of milk Ca into the carcass was significantly slower than that of CaCl2, although by 6 h postintubation both had plateaued at approximately 92% of dose. At 28 days, the time course of transport was the same for the two forms of Ca, and the plateau was not significantly different from that at 14 days. In the younger animals, more Ca from milk than from CaCl2 was retained in gastrointestinal tissue. Quantitation of 45Ca in various segments of the gastrointestinal tract showed that the greater retention of milk Ca occurred in the stomach, the duodenum, and the distal jejunum. We conclude that milk Ca is efficiently absorbed by the suckling rat, possibly in a protein-bound form. The mechanism of the absorptive process awaits further investigation.