The acute effect of prolactin on active calcium absorption was investigated in weaned, young, sexually mature and aged female Wistar rats by using everted gut sac technique. The serosal/mucosal or [S]/[M] ratio of 45Ca concentration was used as an index of active calcium transport. The data showed that intraperitoneal injection of 0.02 mg prolactin/100 g BW, 5 and 10 minutes before experiment could increase the active calcium transport in the duodenum but not other intestinal segments of sexual rats. However, prolactin had no effect on duodenal active calcium transport in weaned, young and aged rats. The mechanism of action prolactin on active calcium absorption was investigated by determining the effect of prolactin on transapical uptake of calcium across the oil-filled duodenal sac in 2 minutes also demonstrated. It was found that intraperitoneal injection of 0.02 mg prolactin/100 g BW, 5 minutes before the uptake experiment could enhance the transapical calcium uptake. These data indicated that acute effect of prolactin on active calcium absorption was nongenomic and not mediated by enhanced production of 1,25(0H)2D3. Prolactin increased active calcium absorption partly by stimulating the transapical calcium uptake across the brush border membrane by as yet enhance mechanism.