The alteration of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver of rats administered orally calcium chloride solution was investigated. The plasma membrane (Ca(2+)-Mg2+)-ATPase activity was significantly increased by a single oral administration of calcium (10, 25 and 50 mg/100 g body weight) in rats. This increase was seen between 10 and 60 min after the administration. The presence of anti-regucalcin IgG (1.0-5.0 micrograms/ml) in the enzyme reaction mixture caused a complete inhibition for the elevation of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity by the addition of regucalcin (0.25 microM). Also, the calcium administration-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was completely abolished by the presence of anti-regucalcin IgG (1.0 and 2.5 micrograms/ml). Moreover, the calcium administration-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was not inhibited by vanadate (0.1 and 0.2 mM) addition into the enzyme reaction mixture, although the inhibitory effect of vanadate was seen in the plasma membranes from normal rat liver. Now, the activating effect of regucalcin (0.25 microM) on hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase was not inhibited by vanadate addition. The endogenous regucalcin may play a role in the calcium administration-induced increase of (Ca(2+)-Mg2+)-ATPase activity in the liver plasma membranes of rats.