1. The GABA antagonist bicuculline has been applied to hypercomplex cells in layers II and III of the cat's striate cortex in an attempt to test the hypothesis that their length preference derives from the action of a GABA mediated post-synaptic inhibitory input.2. Iontophoretic application of bicuculline to these cells resulted in a reduction but not an elimination of the length preference. The reduction in length preference was only observed in the case of slits extended to one side of the receptive field or to slits only partially covering what appeared to be inhibitory flanking regions either side of the field centre. In cells normally showing a clear and stable length preference it was never possible to produce by the application of bicuculline a significant response to a slit fully extended to cover both flanking regions.3. The orientation tuning was basically eliminated by the application of bicuculline. In contrast the directional specificity was relatively unaffected.4. The action of bicuculline on hypercomplex cell orientation tuning supports the view that GABA mediated inhibitory inputs were effectively blocked and suggests that the partial effect on length preference and lack of effect on directional specificity reflect the varying degree of involvement of a GABA mediated inhibitory input to these receptive field properties.5. These observations introduce the possibility that the excitatory input to the superficial layer hypercomplex cells exhibits directional specificity, length preference with respect to a slit extended to both sides of the field and a low degree of orientation selectivity. Evidence is presented indicating that certain layer V cells with hypercomplex type receptive field properties exhibit some of the characteristics required of this input.