Spike trains flowing into the periaqueductal gray (PAG) might be discriminated from one another by PAG neurons on the basis of the distribution or sequence of their respective interspike intervals. The various sequences of interspike intervals characteristic of spontaneous PAG unit activities were assessed in a preliminary experiment. These sequences were then simulated by means of appropriate mathematical functions. These functions allowed the production of stimulation trains that were applied to two PAG sites to induce spike trains with similar sequences in order to reveal the sensitivity of PAG neurons to the stochastic structure of afferent spike trains. We placed emphasis on parameters of the spike train that proved to be altered independently of any alteration of the corresponding parameters in the stimulation train. The mean pulse rate is the simplest example of such a parameter as it was never altered in the stimulation train. Alterations of either the distribution or sequence of pulses in the stimulation train were found to affect the mean discharge rate in a number of cases (30-40% of the cases). Despite their moderate degree (20-30% mean rate alteration) such differential effects could correspond to stimulation-induced differential behavioral effects as was shown in a previous study. Furthermore, a specific dependence of the generated spike trains on the sequential structure of the stimulation train was observed in some cases when appropriate stimulation trains were simultaneously applied to another PAG stimulation site. This fact is worth considering in relation to the integrative function of the PAG neuronal network.