The interaction between 18-methyl norethindrone and ketoprofen,including the displacement of ketoprofen from human serum albumin binding sites,was investigated by the capillary electrophoresis-frontal analysis method (CE-FA)at room temperature.A very large sample plug was introduced hydrostatically into the capillary(65 cm x 50 μm i.d.;effective length of 35 cm)over 80 s at a height difference of 11 cm.The working conditions for CE-FA separation are as follows:operating voltage,10 kV;running buffer,67 mmol.L-1 phosphate,pH 7.4.The unbound ketoprofen concentration was directly measured from the height of the frontal peak.When the concentration of 18-methyl norethindrone was increased from 0 to 200 μmol/L,the unbound ketoprofen concentration was found to increase from 22.4 to 26.4 μmol/L at 100 μmol/L total ketoprofen concentration and from 82.1 to 106.2 μmol/L at 200 μmol/L total ketoprofen concentration.From these data,it may be deduced that the administration of high concentration of 18-methyl norethindrone can displace ketoprofen from its secondary binding site.