Cross-correlations between stimuli and neuronal discharges yield information about synaptic events at the investigated neuron. In this paper it is shown that the time course estimated by a cross-correlogram, the cross-correlation function (ccf), represents the input current that upon injection into the perfect integrator model evokes spike sequences that are (almost) identical to those used for estimation of the ccf. Thus, the shape of a ccf may be regarded as an estimate of the underlying postsynaptic current, if the neuron investigated behaves, at least to a first approximation, like a perfect integrator model.