Changes in the voltage clamp currents of squid giant axons wrought by low axoplasmic TEA+ (tetraethylammonium chloride) concentrations (0.3 mM and above) are described. They are: (a) For positive steps from the resting potential in sea water, the K+ current increases, decreases, then increases, instead of increasing monotonically. (b) For positive steps from the resting potential in 440 mM external K+, the current has an exponentially decaying component, whose decay rate increases with axoplasmic [TEA+]. The control currents increase monotonically. (c) For negative steps from the resting potential in 440 mM external K+, the current record has a peak followed by a decay that is slow relative to the control. The control record decreases monotonically. Qualitatively these findings can be described by a simple kinetic model, from which, with one assumption, it is possible to calculate the rate at which K+ ions move through the K+ channels. An interesting conclusion from (c) is that the channels cannot be closed by the normal voltage-sensitive mechanism (described by Hodgkin and Huxley) until they are free of TEA+.