AbstractDuration of a tachistoscopically presented dark dot was estimated by schizophrenics and alcoholics. The dot was preceded by the auditory warning signal of viariable duration of up to 9 seconds. The duration of the warning signal constituted foreperiod duration. Estimation was a monotonically increasing function of foreperiod duration for both schizophrenics and alcoholics when the probability distribution of foreperiods was constant. However, this monotonicity was eliminated when changes in foreperiod duration became infrequent. When intrusion of extraneous stimulus accompanied such infrequent changes, the effect of foreperiod duration on estimation of stimulus duration became curvilinear, with the maximum estimation at the mid‐range. For alcoholics, the monotonically increasing effect of foreperiod duration was independent of relative frequency of foreperiod changes and intrusion of extraneous stimulus. Weakness and instability of time expectancy as characteristics unique to schizophrenia were demonstrate