A sensitive and rapid HPLC-UV method for in vivo determinations of cocaine levels in extracellular fluid of specific brain regions and plasma is described. Free drug levels resulting from intravenous administration of cocaine were sampled using in vivo microdialysis probes simultaneously located in the jugular vein, nucleus accumbens, and anteromedial caudate-putamen of halothane-anesthetized rats. In a separate group of animals, the influence of cocaine on extracellular dopamine concentrations in the anteromedial caudate-putamen was also assessed. The time dependences of changes in cocaine concentration in each of the above regions were congruent, and peak concentrations were reached 10 min after the drug was administered. The half-lives of cocaine in the blood, nucleus accumbens, and anteromedial caudate-putamen were estimated to be 31.5, 29.1, and 21.4 min, respectively. A repeated injection of cocaine, given 90 min later, produced a maximal cocaine level and pharmacokinetic profile that were indistinguishable from those of the initial infusion. Cocaine was concentrated to a greater extent in brain than in blood, a feature consistent with the action of a lipophilic drug. In addition, extracellular dopamine levels measured in the anteromedial caudate-putamen following cocaine infusions closely mirrored those of cocaine itself. The ability to measure the free concentration of drugs by microdialysis should be applicable to a wide range of in vivo pharmacological studies.