This paper describes a sensitive and reliable method for the determination of cocaine in human urine, plasma and red blood cells. Cocaine is extracted into cyclohexane from the biological materials at slightly alkaline pH, reduced with lithium aluminium hydride, acylated with pentafluoropropionic anhydride and detected by an electron capture detector. When compared with a gas chromatography-mass spectrometry method the results of cocaine determination correlated highly (r = 0.986). When cocaine was given intravenously to volunteer subjects only 0.2-1.4% of the administered dose was excreted as unmetabolized cocaine in the first 9 h after administration. Plasma and red blood cell levels of cocaine were also determined by this method after intravenous administration.