Current meter observations and hydrological and meteorological data collected primarily during 1974 formed the basis upon which the monthly water balance of Georgian Bay could be estimated. The data were used to derive a first approximation of the exchange of water between Georgian Bay and main Lake Huron. The net exchange across Main Channel at depths greater than 5 m was determined from extensive current meter observations while the exchange in the upper 5 m was the residual necessary to achieve a balanced water budget. The results illustrate that during the stratified season (May through November) the transport of water through Main Channel is also stratified with surface water moving into Georgian Bay from main Lake Huron while continuity is maintained by net outflow at depths greater than 5 m. Total annual supply from runoff and direct precipitation is nearly twice as large as combined losses from evaporation and flow to North Channel. Residence and flushing times for water in the bay are estimated at 5.1 and 5.7 years, respectively.