The intergeniculate leaflet (IGL) is a very important component of the mammalian circadian timing system. One of the best known, but still barely understood functions of the IGL, is the integration of photic (retina-derived) and non-photic information, conveyed to the suprachiasmatic nucleus (SCN)--the site of the circadian pacemaker. Glutamate, the main neurotransmitter released from the axonal endings of the retinal ganglion cells to the SCN and most probably to the IGL, is thought to be responsible for mediating the effects of light on the circadian clock. The influence of carbachol, a non-specific cholinergic agonist, on locomotor activity, c-fos expression in the SCN, and the activity of this structure has been previously studied. However, no information is available concerning the influence of acetylcholine on the activity of the IGL neurons. Therefore, the purpose of the present study was to analyze the influence of carbachol (equivalent of non-photic stimulus) on the glutamate-induced activity of the IGL neurons. Experiments were performed on thalamic rat brain slices, using extracellular, single unit recordings. After reaching a stable response to focally applied glutamate, carbachol was added to the recording medium. In the presence of the cholinergic agonist, glutamate-induced activity was decreased in 32% and increased in 13% of investigated cases. Carbachol failed to evoke any change in glutamate-induced activity in 55% of the recording cells. Our results are in agreement with previous, mainly behavioral studies, where the influence of non-photic stimulus on photic-induced changes in circadian locomotor activity was determined.