We studied the effect of DBP on thiopentone hypnosis in 580 patients aged 20 to 30 and 560 patients aged 40 to 50. The hypnotic effect of thiopentone was evaluated after 5, 15, 30, 45 and 60 minutes from intravenous administration of 0.05 mg.kg-1 of DBP by using dose-effect curves to identify the ED50 dose. The ED50 value of thiopentone decreased up to 15 minutes after DBP (about 1.1 mg.kg-1 in both age groups) and increased after 30, 45 and 60 minutes, the increase being higher in the patients aged 40 to 50. We concluded that DBP has a synergic effect with thiopentone and the pharmacokinetic and alpha 1-lytic effects of DBP are responsible of an increased need of barbituric in the patients aged 40 to 50.