Objective To observe the distribution of acetylcholinesterase activity in the thalamus of the monkey.Methods Histochemical method was used to detect the acetylcholinesterase activity in the thalamus.Results Acetylcholinesterase was found to be inhomogeneous distribution in the primate thalamus and to reveal previously uncovered inhomogeneity within certain thalamic nuclei and their subdivisions. The medial, ventral and posterior nuclear groups displayed markedly uneven acetylcholinesterase reaction.In the mediodorsal nucleus,three distinct sbudivisions were revealed by acetylcholinesterase histochemistry, medial magnocellular part, ventral sector of central parvicellular part and dorsolateral sector of lateral pars multiformity showed weak, moderate and strong acetylcholinesterase activity, respectively. In the ventral nuclear group, acetylcholinesterase histochemistry was strong in the medial part of ventral posterior nucleus, moderate in the magnocellular part of ventral anterior, caudal, medial, oral and pars postrema parts of ventral lateral nucleus, as well as lateral part of ventral posterior nucleus, poor and weak in the inferior part of ventral posterior nucleus, par compacta of the medial part of ventral posterior nucleus and parvicellular part of ventral anterior nucleus. In the pulvinar nucleus, acetylcholinesterase reaction ranged from weak, moderate to strong in the parts of the oral, medial and lateral, as well as inferior of this nucleus, respectively. Regional variations of acetylcholinesterase activity within the thalamic nuclei and their subdivisions can help to identify them by acetylcholinesterase histochemistry. In addition, the dark patches of strong acetylcholinesterase activity contrasting with a lighter surrounding matrix were revealed within the parvicellular part and pars multiformis of mediodorsal nucleus, paracentral nucleus, central lateral nucleus, pars postrema part of ventral lateral nucleus and medial habenula nucleus, as well as medial part of pulvinar necleus, respectively.Conclusion Acetylcholinesterase histochemistry can serve as useful marker for delineating the thalamic nuclei and their sbudivisions. The different distribution of acetylcholinesterase histochemistry implies the activity of thalamic nuclei may be modulated by cholinergic systems.