The 112-amino-acid peptide irisin is a product of cleavage and modification from its precursor membrane fibronectin type III domain-containing protein 5 (FNDC5) in response to exercise. Apart from its well-known peripheral effects, such as the ‘browning’ of white adipocytes, irisin can penetrate the blood-brain barrier and induce central effects. In the hypothalamic master clock, an involvement in gating photic stimuli in the retinohypothalamic synapse is suggested for irisin. In the present study on the rat hypothalamic slice preparation, we show for the first time that application of 4 nM irisin induces an increase in firing rate of a substantial proportion (22 of 58 cells recorded: 37.9%) of cells in the suprachiasmatic nucleus (SCN) along with a reduction of the entropy of the log interval histogram, used as a measure of spike irregularity. The proportion of cells showing a decrease in firing rate in the presence of irisin was significantly less (6 of 58 cells recorded: 10.3%) compared to the proportion of cells, in which irisin application induced an increase in firing frequency (p = 0.001, z-test). Irisin did not produce changes in the mutual information between the adjacent log interspike intervals used as a measure of spike patterning. These results show that a substantial proportion of neurones in the circadian clock located in the suprachiasmatic nucleus is sensitive to the direct action of irisin. Currently, the brain receptor for irisin remains unknown. The future discovery of irisin receptor in the brain will give a possibility of identifying the cellular mechanisms of irisin action in the suprachiasmatic nucleus.