The mammalian circadian time-keeping system is established by two major neural structure: the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and the intergeniculate leaflet (IGL) of the thalamus. Both these structures display the ultradian isoperiodic oscillation of the level of neuronal firing which probably constitute a basic activity necessary to generate circadian rhythms. In the present study we discuss the nature, pharmacology and the serotonergic influences on this oscillation in IGL.