The present investigation aimed to (1) reconfirm the acute effect of prolactin on intestinal calcium absorption in the rat, (2) evaluate the influence of age on prolactin action, and (3) further investigate the mechanisms of prolactin action on calcium absorption. The intestinal calcium absorption in mature rats was evaluated by measuring the plasma 45Ca content at intervals during a 1-h period following intragastric administration of a 45Ca-containing test solution and by measuring tibial 45Ca content in weaned rats. By using weaned (50-70 g), sexually mature (180-200g) or aged (greater than 250 g) female Wistar rats, we demonstrated that calcium absorption was inversely related to age. Prolactin at a dose of 0.02 mg/100 g body weight, administered intraperitoneally 1 h before Ca administration, significantly enhanced calcium absorption in sexually mature and weaned rats but not in aged rats. At a luminal calcium concentration of 5 mM, prolactin had no effect when the luminal solution was either CaCl2 alone or an Na-free electrolyte solution. On the other hand, prolactin significantly increased the plasma 45Ca content by 57, 42 and 28% at 5, 15 and 30 min respectively, when the electrolyte solution contained sodium. In contrast, in the presence of 0.5 mM calcium, the enhancing effect of prolactin on calcium absorption was not dependent on the presence of sodium. Compared with the saline control, the plasma 45Ca content in prolactin treated animals was significantly elevated by 63% at 5 min when the test solution was a sodium-free electrolyte solution.