A study was made of the effects of electrophoretically administered bicuculline on the recurrent inhibition of thalamo-cortical relay neurones of the ventrobasal complex and the dorsal lateral geniculate nucleus of decerebrate and anaesthetised cats (pentobarbitone sodium). Electrophoretically administered glycine, GABA and taurine depressed the firing of thalamic neurones. GABA was usually the most potent depressant, glycine the least. The effects of GABA and taurine were blocked by bicuculline, those of taurine and glycine were blocked by strychnine. The inhibition of the firing of lateral geniculate neurones by electrical stimulation of the visual cortex was reduced by bicuculline but not by strychnine. The inhibition of these cells following an optic nerve volley, and the inhibition of ventrobasal neurones by stimulation of the primary somatosensory cortex, were also reduced by bicuculline.