Levels of ethanolamine intermediates in the retina and optic nerves of autopsied human donors and in the rat visual system (retina, optic nerve, lateral geniculate body, superior colliculus) were measured. Amounts were also obtained from the retina, optic nerve, and optic tectum of a primitive elasmobranch, the smooth dogfish Mustelus canis, and from the related nervous structures (retina, optic lobe, fin nerve, and stellate ganglia) of a marine invertebrate, the squid Loligo pealei. In all regions of the human and rat nervous system, the pool size of CDP-ethanolamine (values ranging between 10-31 nmol/g wet wt) was much smaller than that of free ethanolamine (values ranging between 197-395 nmol/g wet wt), whereas glycerophosphorylethanolamine was present in relatively high content (values ranging between 125-280 nmol/g wet wt). In nervous system regions of the dogfish and squid, the distribution of values followed the same general trend as observed for humans and rats, even if all regions had less ethanolamine intermediates compared to the mammalian counterpart. In dogfish and squid retina, glycerophosphorylethanolamine showed the highest pool size among the ethanolamine derivatives analyzed (16 and 44 nmol/g wet wt, respectively). The present study confirms the basic similarity of ethanolamine intermediate pool size patterns in the nervous system structures (with the exception of the retina) of animal species which have widely different phylogenetic positions. The data support the proposal that the levels reached by ethanolamine and its derivatives in the nervous tissue is the result of an ancient evolutionary development of metabolic pathways for the maintenance of phosphatidylethanolamine membraneous content.