Neurons of the mammalian suprachiasmatic nucleus (SCN) generate self-sustained rhythms of action potential frequency having a period of approximately 24 h. It is generally believed that cell autonomous circadian oscillation of a network of biological clock genes drives the circadian rhythm in neuronal firing rate through as yet unspecified effects on the neuronal membrane. While it is clear that cyclic gene expression continues in constant darkness, previous studies have not examined which specific membrane properties of SCN neurons continue to oscillate in constant conditions. Here, we demonstrate that SCN neurons exhibit robust rhythms in resting membrane potential and input resistance in constant darkness. Furthermore, application of the K+ channel blocker tetraethylammonium revealed a rhythm in K+ current amplitude that persists in constant darkness and underlies the rhythm in membrane potential.