The swift response of animal to the applied stimuli suggests the importance of phasic characters of firing impulses in the nervous system. Many mechanoreceptors show rapid adaptation and stochastic discharges. To approach the information processing in such receptor system, we examine the dynamic characteristics of mechanoreceptor units on inner flagellum (antennule) of American crayfish (Procambrus clarkii). The units were characterized by discharging nerve spikes with different magnitudes. The post-stimulus time histogram (PSTH) to sinusoidal wave train (SWT) stimuli was studied by a microcomputer measurement system. When a suitable intensity and interval stimulation is applied, we get peculiar PSTH which can not be explained by usual exponential function only. In order to understand the behavior of PSTH on a unified model, a new idea is proposed on the assumption that firing probability at a period is proportional to the product of normalized recovery function for all previous discharges. The PSTH for SWT stimuli was well explained with the model. General behaviors of PSTH could be understood from the ratio of recovery time constant to the interval between waves of SWT.