Calcium compartments in calf platelets were studied using a lanthanum washout procedure to distinguish between surface-bound calcium and intracellular calcium. The calcium content of calf platelets ranges from 20 to 60 nmol/109. platelets and is sensitive to the calcium concentration of the suspending medium. With 1 mM calcium in the medium, calcium uptake is rapid and reaches steady state within 1-2 min. Results obtained with the lanthanum procedure indicate that it is the surface compartment which is most affected by the extracellular calcium concentration. The surface compartment appears to be saturable and is highly exchangeable. Although the total calcium as well as the calcium content of the surface and internal compartments are variable, the ratio of calcium in either compartment to the total saturated calcium is quite constant. The data indicate that 68-85% of the platelet calcium is located internally. Thrombin produces an immediate release of platelet calcium and labeled serotonin and an increase in the 45Ca2+ uptake of both the surface and internal compartments. The release reaction is not dependent upon exogenous calcium or an influx of exogenous calclium since it occurs even in the presence of ethyleneglycol-bis-(beta-aminoethylether)-N, N'-tetraacetic acid. Lanthanum, however, inhibits the release reaction possibly by blocking surface calcium site and reducing the mobility of endogenous platelet calcium.