The effect of glycerol treatment on the membrane currents and tension development was studied in voltage clamped snake muscle fibers. In muscle fibers which were exposed for 1 h to a normal saline containing 400 mM glycerol and then returned to a normal medium, graded depolarizations did not accompany contractile responses. However, when the fiber was depolarized to a certain level, an increment of outward current appeared which partially inactivated with time. The threshold for delayed rectification in glycerol-treated fibers was almost the same as that of intact fibers in spite of the absence of contractile tension. The results suggest that the delayed rectification may be attributed at least in part to the surface membrane and that the contractile activation probably does not depend simply on the inactivating outward currents through the delayed rectification channel.