The beta-carboline ZK 93,426, a benzodiazepine receptor antagonist, was administered intravenously to human volunteers at two different doses (0.01 mg/kg, 0.04 mg/kg) according to a double-blind, placebo controlled design. Vital functions (i.e. blood pressure, heart rate, ECG, EEG), peripheral (finger) skin temperature and performance in psychometric tests for psychotropic and cognitive effects were evaluated. Blood samples were collected in addition and certain pharmacokinetic parameters were estimated. ZK 93,426 in both doses was well tolerated and exhibited no side effects. A decrease in peripheral skin temperature and heart rate was observed. In a general estimation of behavioural changes, volunteers experienced a stimulant and activating effect of the drug. An improvement in performance was observed in two cognitive tasks, the "logical reasoning task" and "pictures differences task" which estimated concentration and attention, respectively. No effects were found in time estimation. Plasma levels 5 min after intravenous administration of ZK 93 426 were 16 +/- 10 ng/ml and 52 +/- 31 ng/ml for 0.01 mg/kg and 0.04 mg/kg, respectively. Total clearance was calculated as 46 +/- 22 ml/min/kg (0.04 mg/kg).