In order to observe the reaction of cochlear blood flow (CBF) to trimetaphan (TMP)-induced hypotension, CBF was measured with laser-Doppler flowmetry in 7 human subjects during general anaesthesia for middle ear surgery. All subjects showed a decrease in mean arterial pressure (MAP) during intravenous infusion of TMP, followed by a gradual return to the baseline level after termination of the infusion. The CBF generally followed the MAP changes with the same pattern. Three of the seven subjects demonstrated a CBF change larger than the maximum MAP change, indicating the lack of a local autoregulatory mechanism in CBF. On the other hand, CBF changes were smaller in magnitude than the maximum change in MAP for the rest of the subjects, suggesting an autoregulatory mechanism in CBF. However, since the audiograms from these subjects indicated profound damage along the cochlear basal turn probably due to middle ear inflammation, concomitant vascular damage in this region offers another possible explanation for the inappropriate CBF changes. The present observations may also suggest that deliberately TMP-induced hypotension has a potentially harmful effect on CBF during otological surgery that attempts to preserve or improve hearing.