The subject of this article lies within the traditional range of questions which might be designated as pharmacological aspects of invertebrate neu robiology. We will not attempt to cover the entire area of chemical trans mission in invertebrate nervous systems. This review is to call attention to the plurality of the chemical transmission mechanisms within a simple ner vous system and to search for information on the origin of the pharmaco logical heterogeneity of synapses which appears to be a fundamental feature of nervous s ystems including that of man. Consideration wil l be given to re sults which can be interpreted in terms of cellular pharm acology. Emphasis will be on the species best investigated at the unit level, comparative data being drawn in to clarify how far the elected species are representative. It should be noted that a comparative study should not necessarily adopt its assumptions and systems of classification from vertebrate pharmacology and neurology. Cholinergic receptors (ChR) are usually described in terms of vertebrate muscle pharmacology (Muscarinic and nicotin ic recep tors)--,a classification which does not apply to invertebrates (1). The use of inadequate definitions delays the accumulation of data and working out of appropriate classifications of ChR's although some work has been done in this important area (2). Another invalid though widely accepted generaliza tion is the assumption that ACh inactivating enzymes fall into two catego ries, viz., acetylcholinesterase (EC 3.1.1.7) of n eurons, and cholinesterase (EC 3.1.1.8) of glial cells. Comparative data demonstrate that enzymes often do not fit this classification. Thus the single enzyme of the squid brain combines the properties of both mammalian cholinesterases; moreover, brain cholinesterases of decapod and octopod cephalopods are chemically and pharmacologically different enzymes (3-5). Since the presence of an ACh inactivating enzyme at junctional areas is considered to be a relatively late