Varying magnitudes of freshwater input through the St. Lawrence are added to different versions of the GISS coupled atmosphere-ocean model. The results show a generally linear response in percentage North Atlantic Deep Water (NADW) reduction to the volume of water added without any obvious threshold effects. When the estimated best guess freshwater input for the Allerod-Younger Dryas interval is added, only small reductions in NADW production occur in less than a century, with complete cessation requiring more than 300 years. If the ocean circulation was weaker in the Allerod preceding the Younger Dryas, somewhat larger percentage reductions occur, and it would take 150--200 years for a complete shutdown. The freshening is aided by a positive feedback associated with the reduction in evaporation relative to precipitation in the North Atlantic, amounting to some 20--30% of the added freshwater runoff. When the freshwater input is stopped after complete NADW shutdown, NADW production does not resume. With complete NADW cessation, cooling eventually occurs throughout the Northern Hemisphere, including regions where it is not thought to have arisen during the Younger Dryas, and the globe cools by 1 ¿C due primarily to the sea ice albedo feedback; while in many regions the observed cooling is reproduced, the cooling in Greenland is much less than is estimated. With only 50% NADW cessation, cooling is primarily around the North Atlantic. No rapid warming is seen in response to freshwater cessation. The sensitivity displayed here for NADW reduction due to freshwater input from the St. Lawrence is roughly similar to that associated with increased precipitation accompanying global warming in the next century, so a similar sensitivity may arise with Younger Dryas freshwater inputs from other suspected locations (Greenland, Europe). ¿ 2001 American Geophysical Union