Studies dealing with auditory information processing often present the dynamic spectrum of the sound stimulus (sonogram) in addition to the stimulus waveform. The sonogram, presenting the spectral and temporal properties of the sound in a combined way, reflects properties that are assumed relevant in central information processing. For 12 neurons recorded from the midbrain of the grass frog the sonogram of a Gaussian wide-band noise stimulus was correlated with the output of the neuron to that noise. From this input-output correlogram the spectro-temporal receptive field (STRF) was calculated. The STRF reflects those spectral and temporal properties of the stimulus that influence the firing probability of the neuron. A quantitative procedure was developed to calculate the neuron's response as far as it could be derived from the STRF. This procedure basically consisted of a convolution between STRF and the sonogram of the stimulus followed by a summation over the various frequency bands. In this way it proved possible to estimate to what extent the STRF characterised the neuron's firing behaviour. Heuristic approaches, in which the neuron was modelled to a parallel series of band-pass filters, a summator and a static nonlinearity, representing a spike-generating mechanism, resulted in a considerable improvement of the characterisation.