In mammals, the SCN of the hypothalamus contains a circadian (circa day) clock which regulates physiological functions with a period of approximately 24 h (Inouye and Kawamura, 1979; Meijer and Rietveld, 1989). One of the important findings was that the population of isolated and cultured SCN neurons exhibits a circadian rhythm of firing rate that lasts for many days (Welsh et al., 1995; Herzog et al., 1998; Honma et al., 1998), and thus, the circadian rhythms of the SCN neuronal population are a result of synchronized activity of many separate SCN neurons. The circadian clock in a single neuron can be divided into three components: (a) an intracellular circadian clock based on transcription/translation mechanisms and studied thoroughly in the last decade (Reppert and Weaver, 2001; Reppert and Weaver, 2002); (b) cytoplasmic messenger(s) of signals between the intracellular circadian clock and the receptors located on the internal membrane surface; and (c) the receptor-regulated membrane channels which open and close to change the membrane potential and, correspondingly, the firing rate of SCN neurons from zero during the subjective night to a high frequency during the subjective day. The precise nature of all of the cytoplasmic messenger(s) is unknown, and hypotheses regarding the membrane channels responsible for circadian modulation of firing rate are rather contradictory (Jiang et al., 1997; Pennartz et al., 2002; Cloues and Sather, 2003; Kuhlman and McMahon, 2004; Itri et al., 2005; Meredith et al., 2006; Pitts et al., 2006). In the current work, I suggest a hypothesis based on experimental data that SVC channels play a pivotal role in circadian modulation of firing rate in SCN neurons.