Vitamin D-replete (+D) and vitamin D-deficient (-D) rats received large doses of calcium (2-18 mg) by intraperitoneal injection and their responses to the calcium load was analysed in terms of the instantaneous and time-dependent responses of the plasma calcium concentration, [Cas]. Following an initial expansion, [Cas] returned to the preinjection value in a strictly exponential manner, with t1/2 = 22.5 +/- 2.0 (SE) min in +D and 51 +/- 5.2 min in -D animals. In both groups of animals, these rates were independent of the calcium load. Extraprolation of [Cas] to t = 0, i.e., the time just after administration of the calcium, revealed that the amount of calcium circulating at that moment was only about one-fifth of the amount that would have been found if all of the injected calcium had remained in the plasma. Calculations suggest that in all animals about four-fifths of the injected calcium load became distributed virtually instantaneously in the extracellular water. In both +D and -D groups the fraction of the injected load that left the plasma instantaneously was independent of the calcium load, of [Cas] at t = 0 or of the animals' plasma volume. The ability of rats to disperse some 80% of the load to outside the plasma would seem to constitute a major mechanism of acute plasma calcium regulation. Dilution was insufficient, however, totally to reduce [Cas] to the preinjection level. That occurred exponentially, with most of the calcium presumed to enter the skeleton. This exponential rate was markedly and significantly slower in the vitamin D-deficient animals than in their controls.