We provide a functional measure, the synaptic information efficacy (SIE), to assess the impact of synaptic input on spike output. SIE is the mutual information shared by the presynaptic input and postsynaptic output spike trains. To estimate SIE we used a method based on compression algorithms. This method detects the effect of a single synaptic input on the postsynaptic spike output in the presence of massive background synaptic activity in neuron models of progressively increasing realism. SIE increased with increases either in time locking between the input synapse activity and the output spike or in the average number of output spikes. SIE depended on the context in which the synapse operates. We also measured SIE experimentally. Systematic exploration of the effect of synaptic and dendritic parameters on the SIE offers a fresh look at the synapse as a communication device and a quantitative measure of how much the dendritic synapse informs the axon.