l- and d-glutamate, l- and d-aspartate, l- and d-cysteate, l-cysteine sulphinate, and to a considerably lesser extent (if at all), N-acetyl- l-aspartate, excite lateral geniculo-cortical relay cells. 5-Hydroxytryptamine (5-HT) blocks the spontaneous and synaptic firing of lateral geniculate nucleus (LGN) neurones evoked by photic stimulation in concentrations that have little or no effect on the actions of these excitatory amino acids. If this effect of 5-HT reflects postsynaptic antagonism of the optic nerve transmitter on LGN neurones, none of these excitatory amino acids is likely to be the transmitter released from optic nerve terminals. This is supported by the findings that 1-hydroxy-3-aminopyrrolidone-2 (HAP) and l-glutamate diethyl ester (two antaonists of glutamate-like compounds in the central nervous system) are ineffective or far less effective than 5-HT as depressants of the orthodromic firing of LGN neurones. Although it has been reported that the optic nerve of the rabbit contains relatively high levels of ergothioneine, ergothioneine is unlikely to be the transmitter released from optic nerve terminals onto LGN neurones, as it has no clear excitant action on lateral geniculo-cortical relay cells. In the medial geniculate nucleus (MGN). 5-HT does not have selective, dose-dependent depressant actions on the different forms of excitation, but probably depresses firing exclusively by a postsynaptic mechanism of action. The excitatory transmitter released from auditory afferent fibers onto MGN neurones and that released from medial lemniscal fibres onto relay cells of the ventrobasal thalamus is/are different from the optic nerve transmitter operating on LGN neurones.