One striking transformation in response properties that occurs in the geniculo-cortical pathway is the appearance of a high degree of orientation selectivity in the cortex. This property may be conceived as arising purely from the excitatory inputs to the cell, as being structured largely by the inhibition a cortical cell receives or could be due to a combination of the two. We have studied the contributions of excitatory and inhibitory inputs to cortical cells' orientation selectivity by analyzing the postsynaptic potentials evoked in cat striate neurones by flashing stimuli of different orientations. We made these recordings using the in vivo whole-cell technique (Xing Pei et al., 1991), which provides more stable and reliable results than classical intracellular recording methods. Our results show that the cat striate cortex exhibits a variety of mechanisms to achieve orientation selectivity. Orientation selectivity of a particular cell can be created by excitatory, by inhibitory, or by a combination of both mechanisms.