“…Two results suggest that none do: (a) the excitation elicited by glutamate but not by the more potent agonists DLH and cis-ADCP is preferentially antagonized by GDEE while the converse is true for DAA which spares the glutamate-induced excitations, and (b) the time courses of onset of excitation for the cyclic amino acids are very much longer than are those for glutamate and aspartate. Although it is possible that the long latency to onset of effect of some of these compounds indicates that a conformational change has been induced in the receptor by them, analogous to the situation for some opiate binding processes (Copeland & de Barre, 1976), taken in conjunction with the data derived from the use of antagonists, the concept that different populations of receptors are involved seems probable. In this connexion too it may be noted that Engberg, Flatman & Lambert (1975) and Altmann, Bruggencate, Pickelmann & Steinberg (1976) have reported that the conductance changes elicited in motoneurones by DLH and by glutamate are not identical.…”