The intergeniculate leaflet of the thalamus (IGL) is a part of the mammalian biological clock which integrates photic and non-photic information and conveys it to the master biological clock - suprachiasmatic nuclei (SCN). One of the non-photic cues is delivered by the serotoninergic projection from dorsal raphe nucleus. In vitro electrophysiological recordings were performed from single IGL neurons using whole-cell patch clamp technique. We investigated the influence of serotonin (serotonin creatinine sulfate complex, 5HT) on 'spontaneous' neuronal activity in this structure. In most of recorded cells, 5-HT caused significant increases in firing rate. In majority of cases the effect was presynaptic. However, in some cases we observed postsynaptic depolarization. To our knowledge, depolarizing influence of 5HT on the single neurons in the IGL has been shown here for the first time.