The presence of internuclear neurons in the abducens and oculomotor nuclei of lampreys [Gonzalez, M.J., Pombal, M.A., Rodicio, M.C. and Anadon, R., Internuclear neurons of the ocular motor system of the larval sea lamprey, J. Comp. Neurol. 401 (1998) 1-15] indicates that coordination of eye movements by internuclear neurons appeared early during the evolution of vertebrates. In order to investigate the possible involvement of inhibitory neurotransmitters in internuclear circuits, the distribution of γ- aminobutyric acid (GABA) in the extraocular motor nuclei of the lamprey was studied using immunocytochemical techniques. Small GABA-immunoreactive (GABAir) neurons were observed in the three ocular motor nuclei. Numerous GABAir neurons were observed in the group of internuclear neurons of the dorsal rectus oculomotor subnucleus. A second group of GABAir neurons was observed among and below the trochlear motoneurons. Two further groups of GABAir interneurons, periventricular and lateral, were located in the abducens nucleus among the cells of the caudal rectus and the ventral rectus motor subnuclei, respectively. In addition to the presence of GABAir neurons, in all the ocular motor nuclei the motoneurons were contacted by numerous GABAir boutons. Taken together, these results suggest that GABA is involved as a neurotransmitter in internuclear pathways of the ocular motor system of lampreys. (C) 2000 Elsevier Science B.V.