The influx and efflux kinetics of 45Ca2+ were studied in the rat lens in vitro. Both data sets could be fitted by a multi-compartment mathematical model and were interpreted in terms of extracellular, cytosolic and slowly-exchanging (bound) components. At the end of a 16-hr influx period, when uptake into the extracellular and cytosolic compartments is complete, the 45Ca2+ exchanged fraction is less than 20% of the total calcium determined by atomic absorption. The bound compartment is therefore by far the largest in the lens. The efflux rate constant determined from the model for the cytosolic compartment was approximately 8 x 10(-3) min-1 and its origin was confirmed by its sensitivity to temperature, absence of external sodium and presence of the amiloride-analogue, dichlorobenzamil. A 55% reduction in efflux was obtained in sodium-free solution, indicating that Na(+)-Ca2+ exchange is responsible for a large proportion of calcium movement from the lens against its electrochemical gradient. This was confirmed in influx studies where, reduction of the lens sodium gradient by either exposure to sodium-free medium or 0.1 mM ouabain significantly elevated the 45Ca2+ content of the lens relative to the control level. Exposure to sodium-free conditions also rendered the lens opaque, which did not occur in the absence of external calcium. These experiments suggest a critical role for Na(+)-Ca2+ exchange in maintaining a low internal Ca2+ and hence transparency.