In a dendritic tree transient synaptic inputs activating ionic conductances with an equilibrium potential near the resting potential can veto very effectively other excitatory inputs. Analog operations of this type can be very specific with respect to relative locations of the inputs and their timing. We examine with computer experiments the precise conditions underlying this effect in the case of b-like cat retinal ganglion cell. The critical condition required for strong and specific interactions is that the peak inhibitory conductance change must be sufficiently large almost independently of other electrical parameters. In this case, a passive dendritic tree may perform hundreds of independent analog operations on its synaptic inputs, without requiring any threshold mechanism.