AbstractVoltage‐gated K+ channels play important roles in shaping the characteristics of action potentials and electrical activity. In a previous study, we isolated cDNAs encoding several distinct K+ channel isoforms, including a novel isoform (XKv1.10) expressed in Xenopus laevis spinal cord neurons and myocytes. Here, we report the biophysical characterization of XKv1.10 expressed in transiently transfected HEK293 cells. Whole cell patch clamp recordings revealed a voltage‐gated, rapidly activating and inactivating K+ current. Interestingly, the rate of inactivation of XKv1.10 channels showed apparent voltage dependence, with time constants between 77.7–213.3 ms. The predicted protein sequence of XKv1.10 does not appear to encode an N‐terminal inactivating “ball and chain” domain, and instead these channels may inactivate via a C/P‐type mechanism. Consistent with this, either increasing the external concentration of K+ or external application of tetraethylammonium caused a decrease in the rate of inactivation. Pharmacologically, XKv1.10 K+ channels were sensitive to 4‐aminopyridine and tetraethylammonium with apparent IC50 values of 68.5 μM and 17.1 mM, respectively. When simulated action potentials were used as a voltage command, XKv1.10 was similar to XKv1.4 in that it carried more repolarizing current during the action potential than XKv1.2. However, while XKv1.4 was active during the interspike interval, XKv1.10 and XKv1.2 were not. Overall, the data suggest that XKv1.10 channels make a unique contribution to the developmental maturation of electrical signaling in Xenopus laevis. © 2004 Wiley Periodicals, Inc. J Neurobiol 60: 227–235, 2004