Effects of oral alprazolam (0.5 and 1 mg) and lorazepam (2 mg) on sympathetic adrenomedullary activity and sedation were studied during supine rest, mental load (Color Word Test, CWT) and active standing (OCT), in 12 male volunteers in a randomized double-blind placebo-controlled cross-over design. Compared to placebo, alprazolam significantly increased subjective sedation, reduced plasma adrenaline and noradrenaline concentrations and mean blood pressure (MBP) during supine rest, and attenuated plasma adrenaline responses during the CWT and the OCT; these effects during the CWT and OCT appeared to be dose-dependent. In comparison with lorazepam (2 mg), alprazolam (1 mg) showed reduced MBP levels during supine rest, whereas lorazepam showed a higher heart rate level during supine rest, a reduced plasma noradrenaline response to the OCT and a performance deterioration to the CWT. There were no differences between alprazolam (1 mg) and lorazepam regarding subjective sedation. Although the benzodiazepines were similar regarding their increase of sedation, alprazolam and loraze- pam induced differential effects on sympathetic adrenomedullary activity during rest and stress, whereby suppression of adrenomedullary activity may be specific for alprazolam.