Postsynaptic inhibition can operate by two distinct mechanisms: (1) membrane hyperpolarization and (2) shunting of excitatory postsynaptic currents. The arithmetic operations--either addition or multiplication-- that synapses are able to perform during neuronal computations are determined by which of these two inhibitory mechanisms predominates. Hyperpolarizing IPSPs interact linearly with EPSPs; their negative and positive synaptic currents sum to produce a net change in membrane potential (Eccles, 1961). Shunting synapses interact nonlinearly with EPSPs; the shunt-induced increase in membrane conductance directly reduces the amplitude of EPSPs by a constant multiplicative factor (Fatt and Katz, 1953; Blomfield, 1974). This property of shunting inhibition has provided the basis for models of synaptic interaction in which shunting inhibition acts as an AND-NOT gate for excitatory inputs (Torre and Poggio, 1978; Koch et al., 1983). Using an in vivo variant of the whole-cell patch technique (Blanton et al., 1989), we have examined the effect of visually evoked inhibition on the size of EPSPs in cortical simple cells and found that the predominant inhibitory mechanism is hyperpolarization. We conclude that these inhibitory synapses operate primarily in the linear mode.