The release of sucrose into the vessel water of sugar maple (Acer saccharumMarsh.) has been investigated with biochemical and enzyme-cytochemical methods. The amount of sucrose released into the vessels in isolated stem segments was found to be temperature-dependent. The release of sucrose also proved to be inhibited whenp-chloromercuribenzoate as an inhibitor of respiratory enzymes was present in the vessels. This suggests that the sucrose release is dependent upon the respiratory activity of some cells. In the cytochemical studies much increased activity of respiratory enzymes (succinate, NAD-dependent isocitrate, and alcohol dehydrogenases) and of acid phosphatases could be demonstrated in the contact cells of the ray and axial parenchyma. Phosphatase activity was concentrated on the large pits between contact cells and vessels. These increased enzyme activities in contact cells were restricted to the time when sucrose appeared in the vessels. Since contact cells are the only cells of the ray and axial parenchyma that show peculiar pit connections with the vessels and also show increased respiratory and phosphatase activity, these cells are considered to be the specific sites of a metabolically controlled sucrose release.