In the present study, a novel, fast, sensitive and robust method to quantify clozapine in human plasma using quetiapine as the internal standard (IS) is described. The analyte and the IS were extracted from plasma using a single protein precipitation extraction technique with methanol and analyzed by high performance liquid chromatography coupled to the electrospray ionization tandem mass spectrometric (HPLC-ESI-MS/MS). The method was linear over the range 20 to 1500 ng.mL-1. The intra-assay precisions ranged from 3.8 to 5.9%, while inter-assay precisions ranged from 4.2 to 6.0%. The intra-assay accuracies ranged from 99.3 to 107.5%, while the inter-assay accuracies ranged from 98.9 to 101.7%. This method agrees with the requirements proposed by the US Food and Drug Administration of high sensitivity, specificity and high sample throughput and was used to evaluate the pharmacokinetic profiles and bioequivalence of the two clozapine formulations in twenty six schizophrenic patients affected by refractory schizophrenia under steady-state conditions. During the hospitalization period the patients received the 100 mg clozapine formulation tablets corresponding to the same dose they were using 14 days before hospitalization. The clozapine pharmacokinetic did not differ significantly after administration of both test and the reference formulations. The Tmax and T1/2 for the test formulation were 2.26 and 10.92 h, respectively. In addition, the Tmax and T1/2 for the reference formulation were 2.44 and 11.08 h, respectively. The 90% confidence interval of the mean ratio of lnAUC0-t was within 0.80-1.25 range which indicates that the test formulation was bioequivalent to the reference formulation when orally administered to schizophrenic patients regarding both the rate and extent of absorption.