1. We examined the hypothesis that the ascending reticular arousal system influences thalamic transmission through a cholinergic mechanism. Extra- and intracellular recordings were obtained from neurons of the dorsal lateral geniculate nucleus (LGNd) and the perigeniculate nucleus (PGN) of cats anesthetized either with N2O and pentobarbital or with N2O and halothane. We compared the effects that electrical stimulation of the mesencephalic reticular formation (MRF) and ionophoretically applied acetylcholine (ACh) have on spontaneous and evoked activity of individual neurons and tested whether these effects could be antagonized by ionophoretic administration of the muscarinic receptor blocker scopolamine. The effects of ionophoretically applied glutamate (GLU), N-methyl-D-aspartate, and bicuculline were examined in addition. 2. The prominent effects in LGNd relay cells of both ACh application and of MRF stimulation were an enhancement of the resting discharge, a facilitation of the excitatory responses to light, a reduction of the amplitude and duration of evoked inhibitory episodes, and a blockade of postinhibitory rebound burst. These latter effects resembled those induced with bicuculline. Under barbiturate anesthesia neither ACh application nor MRF stimulation elicited discharges when the excitatory input from the retina was blocked. Ionophoretic application of hte muscarinic antagonist scopolamine abolished the effects of ACh ionophoresis in all relay cells tested (n = 20), and in 10 cells it also antagonized completely the effects of MRF stimulation. In the remaining cells scopolamine reduced the effects of MRF stimulation. 3. Increasing the depth of anesthesia reduced or abolished the effects of ACh application and MRF stimulation on the cells' resting activity but did not interfere with the facilitation of evoked responses. 4. The effects of the excitatory amino acids GLU and NMDA differed from those of MRF stimulation and ACh application, since the former always enhanced both spontaneous and evoked discharges but neither shortened phases of evoked inhibition nor abolished postinhibitory rebound bursts. 5. There was a high correlation between the effectiveness of MRF stimulation and ACh application in individual neurons. On the average, the facilitation of evoked responses was more pronounced in X- than in Y-cells, and the fraction of cells responding with an increase of resting activity to both procedures was considerably higher among X- than among Y-cells.(ABSTRACT TRUNCATED AT 400 WORDS)