1. The mechanisms of action of acetylcholine (ACh) in the medial (m.g.n.) and dorsal lateral geniculate (l.g.n.d.) nuclei were investigated using intracellular recordings techniques in guinea-pig and cat in vitro thalamic slices. 2. Application of ACh to neurones in guinea-pig geniculate nuclei resulted in a hyperpolarization in all neurones followed by a slow depolarization in 52% of l.g.n.d. and 46% of m.g.n. neurones. Neither the hyperpolarization nor the slow depolarization were eliminated by blockade of synaptic transmission and both were activated by acetyl-beta-methylcholine and DL-muscarine and blocked by scopolamine, indicating that these responses are mediated by direct activation of muscarinic receptors on the cells studied. 3. The ACh-induced hyperpolarization was associated with an increase in apparent input conductance (Gi) of 4-13 nS. The reversal potential of the ACh-induced hyperpolarization varied in a Nernstian manner with changes in extracellular [K+] and was greatly reduced by bath application of the K+ antagonist Ba2+ or intracellular injection of Cs+. These findings show that the muscarinic hyperpolarization is mediated by an increase in K+ conductance. 4. The ACh-induced slow depolarization was associated with a decrease in Gi of 2-15 nS, had an extrapolated reversal potential near EK, and was sensitive to [K+]o, indicating that this response is due to a decrease in K+ conductance. 5. In contrast to effects on guinea-pig geniculate neurones, applications of ACh to cat l.g.n.d. and m.g.n. cells resulted in a rapid depolarization in nearly all cells, followed in some neurones by a hyperpolarization and/or a slow depolarization. The rapid excitatory response was associated with an increase in membrane conductance, had an estimated reversal potential of -49 to -4 mV and may be mediated by nicotinic receptors. The hyperpolarization and slow depolarization were similar to those of the guinea-pig in that they were associated with an increase and decrease, respectively, of Gi, and were mediated by muscarinic receptors. 6. The muscarinic hyperpolarization interacted with the intrinsic properties of the thalamic neurones to inhibit single-spike activity while promoting the occurrence of burst discharges. The muscarinic slow depolarization had the opposite effect; it brought the membrane potential into the range where burst firing was blocked and single-spike firing predominated. Depending upon the membrane potential, the rapid excitatory response of cat geniculate neurones could activate either a burst or a train of action potentials.(ABSTRACT TRUNCATED AT 400 WORDS)