Figure 1. The effect of increasing unbound plasma alfentanil concentration on the core temperatures triggering sweating (open circles), vasoconstriction (filled circles), and shivering (open triangles) in eight volunteers. Unbound alfentanil, up to 30 ng/ml plasma concentration, linearly reduced both the vasoconstriction and shivering thresholds by 0.075 plus/minus 0.067 and 0.063 plus/minus 0.037 degree Celsius *symbol* ng sup -1 *symbol* ml, respectively. The sweating threshold increased only slightly (0.021 plus/minus 0.016 degree Celsius *symbol* ng sup -1 *symbol* ml). All the thresholds (at a designated skin temperature of 34 degrees Celsius) were calculated from measured skin and core temperatures.