The effect of glucose on acute 45Ca uptake and efflux in collagenase-isolated islets was studied using a double-isotope incubation technique with [3H]sucrose as an extracellular marker. Both 45Ca uptake (0-70 min) and efflux (0-80 min) were measured in either 3 or 20 mM glucose. Calcium-45 uptake and efflux were biphasic demonstrating a rapid phase (0-1 min) followed by a slow phase. Glucose (20 mM) increased the rate constant for slow-phase 45Ca uptake 7-fold and had no effect on the rapid phase. Suppression of insulin release by D2O (100%) did not affect glucose-induced 45Ca uptake indicating that this increased uptake occurred independent of insulin release. Rapid-phase and slow-phase 45Ca efflux rate constants were unaltered by 20 mM glucose and inhibition of insulin release by D2O did not influence 45Ca efflux. The rapid-phase movement of 45Ca may represent a rapidly exchangeable calcium pool at the cell membrane whereas the slow phase may be transmembranous calcium movement, as has been reported for calcium transport in HeLa and kidney cells.