1 The effects of bicuculline on dorsal and ventral root activity and upon the depressant effect of gamma-aminobutyric acid (GABA) and glycine on ventral root responses have been studied on the isolated spinal cord of the frog.2 In the absence of stimulation, the alkaloid induced a variety of activity of which the most notable was phasic simultaneous slow wave depolarization in the dorsal and ventral roots which could be reduced or suppressed by magnesium.3 With low concentrations of bicuculline, the adjacent dorsal root response evoked by a single stimulus was depressed maximally before an increase in the ventral root response could be discerned.4 The bicuculline-induced dorsal root activity (in the absence of stimulation) was still apparent at times when the evoked dorsal root response was reduced.5 Bicuculline did not differentiate between the depressant effects of GABA and glycine on the evoked ventral root responses.6 The excitant effects of bicuculline reported here did not appear to be attributable to specific antagonism of the postsynaptic depressant action of GABA.