Spontaneous discharges of individual neurons in the suprachiasmatic nucleus (SCN) of Clock mutant mice were recorded for over 5 days in organotypic slice cultures and dispersed cell cultures using a multi-electrode dish. Circadian rhythms with periods of about 27 hours were detected in 77% of slice cultures and 15% of dispersed cell cultures derived from Clock/Clock homozygotes. These findings indicate that the Clock mutation lengthens the circadian period but does not abolish the circadian oscillation, and suggest an important role of intercellular communication in the expression of circadian rhythm in the SCN.