The distribution of methionine-enkephalin-like structures in the cat thalamus after electroacupuncture was studied using an indirect immunoperoxidase technique. We have observed some alterations on the distribution of methionine-enkephalin-like fibres in the thalamus of the cat after electroacupuncture. Thus, in the nuclei habenularis lateralis and lateralis dorsalis, in the tractus mamillo-thalamicus and in the tractus habenulopeduncularis, we have found methionine-enkephalin-like fibres in the animals treated with electroacupuncture, however in such nuclei and tractus no immunoreactivity was observed in the control animals. On the contrary, after electroacupuncture we have not found methionine-enkephalin-like fibres in the thalamic nuclei centrum medianum, parascicularis and subparafascicularis, in which in the animals no pretreated with electroacupuncture, immunoreactive fibres were observed. In other thalamic nuclei such as habenularis medialis, paraventricularis anterior, centralis medialis, medialis dorsalis and in the stria medullaris no alteration on the distribution of immunoreactive fibres was observed in the animals treated with electroacupuncture in comparison with the distribution of such fibres found in the control animals. These observations suggest that the electroacupuncture might induce alterations on the distribution of methionine-enkephalin-like in certain nuclei of the thalamus of the cat.