We carried out a systematic study on the effects of elevating [K]Oon the properties of a transient outward potassium channel encoded by a cardiac cDNA (RHKI) and compared them with those on twoShakerpotassium channels (H-4 and H-37). The amino acid sequences of all three channels are known, and their structure-function relations have been partially characterized. All three channels were expressed inXenopusoocytes and studied under double-microelectrode voltage-clamp conditions. For all three channels, elevating [K]ocaused an increase in the channels' chord conductances and a negative shift in the calculated activation curves. However, in other aspects of channel properties that are related to the channels' inactivation processes, there were differences in the changes induced by increasing [K].: 1) Elevating [K]. caused a positive shift in the steady-state inactivation curves of RHK1 and H-4 but did not cause any shift in H-37.2) Elevating [K]. slowed the time course of inactivation of H-37 but did not cause any significant changes in the time course of RHK1 or H-4. 3) Elevating [K]0accelerated the rate of recovery from inactivation of RHK1 and H-4 but slowed the recovery time course of H-37. Our experiments show that elevating [K]0can cause a wide range of effects on the transient outward potassium channels. Furthermore, raising [K]0induced similar changes in RHK1 and H-4 (inactivation mediated by an “N-type” mechanism) that were different from the changes in H-37 (inactivation mediated by a “C-type” mechanism). Therefore, our data suggest that part of the effects of elevating [K]0on channel properties may depend on the channel's inactivation mechanism. This hypothesis is supported by results from experiments studying the effects of elevating [K]0on a mutant RHK1 channel (RHK1A3-25), which apparently lacks the N-type and C-type inactivation mechanisms.