Previous studies suggested that slow administration of propofol leads to reduction of dose necessary if titrated to effect.[1] This study investigates the maximum effect of a propofol bolus (2 mg/kg) on BIS if given with different rates. After HIC approval and informed written consent 99 unpremedicated patients (18-60 y) with normal weight were randomized to three groups, receiving an induction bolus of propofol (2 mg/kg). In group 1, this bolus was given over 5 sec, in group 2 over 120 sec, and in group 3 over 240 sec. BIS was used to measure the hypnotic effect of propofol. X2 and ANOVA was used to compare demographic data and BIS minimum between groups. There were no differences in demographic or baseline data. In group 1 (5 sec) the BIS minimum was 28,66 (± 10,29), in group 2 (120 sec) 32,98 (± 13,92) and in group 3 (240 sec) 36,37 ( ± 10,99). The results of group 1 compared to group 3 showed significant differences. The results of group 2 did not show significant differences compared to group 1 or 3 (p<0.05, Bonferroni-corrected). Rapid administration increases hypnotic effect of a propofol bolus as measured by BIS. Conflicting results of previous studies (1,2) are most likely due to differences in study design: If propofol is titrated to clinical effect, slow administration seems to decrease dose requirements. This is most likely due to improved titration. If the same amount is given with different rates, propofol leads to deeper hypnosis if administered faster. These results are consistent with pharmacokinetic models developed in sheep. [2, 3] [References: 1. Anesth Analg 1991. 72: 578-83, 2. Br J Anaesth, 1997. 79: 505-13, 3. Br J Anaesth, 1997. 79(4): 497-504.]