A high-performance liquid chromatographic (HPLC) assay was developed for the quantitation of two structurally similar and highly active anticancer drugs, etoposide (I) and teniposide (II), and their potential metabolites (hydroxy acid, picrolactone, and aglycone). The assay utilizes electrochemical detection, which imparts specificity and sensitivity sufficient to detect greater than or equal to 20 ng/mL in plasma, urine, and CSF. The mean assay coefficients of variation were 5.1 and 8.1% for teniposide (10 micrograms/mL) and etoposide (5 micrograms/mL), respectively. The extraction efficiencies were 86% for etoposide, 70% for its hydroxy acid metabolite, 66% for teniposide, and 54% for the hydroxy acid of teniposide. The correlation coefficient of the multilevel standard curve was greater than or equal to 0.995 over the concentration range of 0.05-50 micrograms/mL for the parent drugs and metabolites extracted from plasma. This method has been used to determine the concentrations of the parent drugs and their metabolites in the plasma, urine, and CSF of patients with cancer.