The slow oscillation circuit of the intergeniculate leaflet seems to constitute a natural basic rhythm of the neuronal mechanism of mammalian biological clock. The results of studies conducted so far indicate that photic information flowing from ganglion cells of the retina is necessary for its generation. On the other hand, this circuit is maintained thanks to the oscillatory activity of GABAergic interneurons, the majority of which build up this structure, mainly in combination with neuropeptide Y and enkephalins. The activity of non-specific projections of the brain, whose terminals are present in the intergeniculate leaflet, modulates the slow oscillation circuit of the leaflet neurons, without changing its oscillatory pattern, though. Our hypothesis predicts a role of the oscillatory activity of intergeniculate leaflet neurons in facilitation the secretion of neuropeptides and neurohormones present in the very elements making up the mechanism of mammalian biological clock and structures linked to it. This constitutes a kind of functional junction between the central mechanism of mammalian biological clock with an ultradian rhythm and its peripheral clocks whose rhythm is circadian.