Figure 1. Mean plasma concentrations of propofol (top) and alfentanil (bottom) over time in the four treatment groups, A-D. The mean plasma concentrations of alfentanil in the four treatment groups at 15 min (when cutaneous nerve stimulation was applied) were A = 0 ng/ml; B = 41 +/- 17 ng/ml; C = 113 +/- 54 ng/ml; D = 130 +/- 61 ng/ml. The mean plasma concentrations of alfentanil at 45 min when the second assessment of adequate anesthesia was made were 0, 40 +/- 17, 102 +/- 47, and 131 +/- 42 ng/ml. There were no significant changes in concentration over time. Propofol concentrations in plasma were deliberately varied over time in response to patient requirements and reduced by 25% at 30 min. There were no significant differences in mean plasma concentrations from 10–20 min, except in group B, in which the concentration at 20 min (39 ng/ml) was greater than the concentration at 10 min (35 ng/ml), P = 0.0147. Similarly, there were no differences in mean plasma concentrations from 40–50 min. Final plasma propofol concentrations (when nitrous oxide was turned off) were 4.9 +/- 2.6, 2.7 +/- 1.5, 1.4 +/- 0.8, and 1.1 +/- 0.6 [micro sign]g/ml. The actual time when nitrous oxide was stopped varied depending on duration of surgery. For graphical reasons, data are presented with +/- SE bars in the figure.