An investigation was made of the influence of strychnine on the depression of the firing of spinal interneurones and Renshaw cells by glycine, GABA, nor-adrenaline and 3-hydroxytyramine. Administered electrophoretically or intravenously, strychnine blocks the effect of glycine more readily than that of the other depressants. Such specific antagonism of glycine action by relatively low concentrations of strychnine may be competitive in nature, but technical difficulties precluded a full assessment of the type of antagonism. The effects of relatively high concentrations of strychnine on the action of the other depressants probably result from interference with membrane permeability changes. The findings are considered to support previous proposals that glycine is the transmitter at spinal strychnine-sensitive inhibitory synapses.