Contrary to expectations based on heightened solution viscosity, alamethicin channels appear to speed up in the presence of water soluble polyethylene glycols (PEGs) and dextrans. Specifically, added polymers reduce the probabilities of transition to higher-conductance states but do not change channel lifetimes. They thereby shorten the duration of current "bursts." These modified probabilities and kinetics reveal the action of polymer osmotic stress to suppress channel formation. The osmotic action of large, fully excluded polymers shows that some 3,000 A3 of water are taken up by the channel from the solution upon each transition to an adjacent higher-conductance state. The partial osmotic action of incompletely excluded polymers reveals the extent of exclusion for different-size polymers. The partial exclusion thus measured agrees remarkably well with estimates using data on reduction of single-channel conductance by current-impeding polymers. One can relate the degree of each polymer's exclusion to its size and to the radius of the channel pore.