The cochlear microphone (CM) has been viewed as stable over long period of time. Recent experiments in which CM was used as an index of hearing loss noted changes occurring as a function of time alone. Experiments were therefore conducted to study these changes. Shifts in CM sensitivity were measured at 13 frequencies in 13 anesthetized cats for periods of up to 300 min. The animals became less sensitive as a more or less linear function or time, the average shifts amounting to about 4 dB. The shifts are regular enough that data can be corrected to allow for them: however, the cause of the shifts their eventual size, and their reversibility all remain to be established.