Listeners attempted to discriminate sounds that differed in the rate at which frequency changed from an initial to a final value, in sound bursts that were much longer than the frequency transition itself. The results indicate that the rate of change of frequency for which the discriminability is optimum depends upon the extent of frequency change, from the initial and final frequencies. For small frequency differences, the optimum appears at small glide rates—i.e., at transition durations in the range of seconds; for large frequency differences, the optimum discriminability occurs at high glide rates, at transition durations of some tens of milliseconds. It seems that there are two mechanisms involved in hearing for perception of frequency transitions.