The effects of i.v. injection of diazepam 0.3 mg/kg and thiopental 6.0 mg/kg on vestibulo-ocular reflex (VOR) were studied in 9 healthy volunteers. One hour after injection of both diazepam and thiopental VOR gain was reduced and time constant shortened. Four hours after intravenous injection of diazepam the VOR gain was still significantly reduced and had not yet returned to control 8 h after injection. Although both drugs were detected in blood still 8 h after the injection, there were no longer significant effects on VOR. Moreover, individual values of VOR gain and time constant did not correlate with the blood concentrations of the drugs. Decrease of VOR gain and shortening of time constant after i.v. given diazepam and thiopental in man are interpreted as resulting from both specific effects on VOR and non-specific reduction of alertness.