Figure 2. The sweating threshold increased as a function of unbound plasma meperidine concentration: Sweating = 0.5[meperidine (degree Celsius [center dot] micro gram sup -1 [center dot] ml)]+ 37.1, r²= 0.10. In contrast, meperidine produced a linear decrease in the core temperature triggering vasoconstriction: Vasoconstriction =-3.0[meperidine (degree Celsius [center dot] micro gram sup -1 [center dot] ml)]+ 36.6, r²= 0.54. Meperidine decreased the shivering threshold nearly twice as much as the vasoconstriction threshold: Shivering =-5.6[meperidine(degree Celsius [center dot] micro gram sup -1 [center dot] ml)]+ 35.6, r²= 0.62. Dashed lines indicate 95% confidence intervals. These regression slopes differ from those reported in Figure 1because they were calculated from the combined values in all volunteers rather than from individual data.