The mammalian master clock driving circadian rhythmicity in physiology and behavior resides within the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Circadian rhythms are generated by a set of clock genes via intertwined negative and positive autoregulatory transcription-translation feedback loops. The Cryptochrome 1 and 2 genes are indispensable for molecular core oscillator function, as evident from the arrhythmic wheel-running behavior and lack of rhythmic clock gene expression in mCry1/mCry2 double-mutant mice in constant darkness. In the present study, using real-time multiunit electrode activity recordings in hypothalamic slices, we show that SCN neurons from mCry-deficient mice kept in constant darkness lack circadian oscillations in firing patterns. This proves that cryptochromes, and thus an intact circadian clockwork, are prerequisites for circadian electrical activity in SCN neurons. Interestingly, when mCry-deficient mice were kept in normal light-dark conditions and SCN slices were prepared 2 hr after the beginning of the day, a single noncircadian peak in neuronal activity was detected. This light-induced rise in electrical activity of the SCN may explain why mCry-deficient mice lack the arrhythmic short bouts of wheel-running activity and instead show apparently normal behavior in normal day-night cycles.