In order to study the influence of labetalol on the peroperative oxygenation during deliberate hypotension, 50 patients undergoing middle ear microsurgery were randomly divided in two groups of 25 patients; one group receiving labetalol to induce hypotension, the other group receiving placebo in a double blind manner. In the group receiving labetalol the arterial oxygen tension (PaO2) decreased significantly 5 minutes after the intravenous bolus injection. Within 30 minutes the PaO2 returned to the starting level. The arterial carbon dioxide tension (PaCO2) increased within 5 minutes and also returned to the initial level after 30 minutes. These changes were absent in the control group. It is concluded that, although labetalol produces a statistically significant change in PaO2 and PaCO2, this finding has only minor clinical implications and so labetalol remains a safe drug to be used for the induction of deliberate hypotension during middle ear microsurgery.