4-[4"-(2",2",6",6"-tetramethyl-1"-piperidinyloxy) amino]-4'- demethylepipodophyllotoxin (GP-7) is a new podophyllotoxin spin-labeled derivative. Its primary effect is the antitumor activity on transplanted mouse tumors and cultured tumor cells. This paper describes a method for its determination using HPLC with UV detection and the determination of its pharmacokinetic parameters in rats. A Shimadzu LC-6A liquid chromatograph equipped with a Shimadzu SPD-6AV multiwavelength detector and a Chromatopac C-R3A data processor was used. The separation was performed on a Zorbax-ODS column (5 microns, 4.6 mm x 150 mm) with a mobile phase of methanol--water--glacial acetic acid (59:41:0.6). The flow-rate was 1.0 ml.min-1 and detection was made at 285 nm. A plasma specimen (0.2 ml) was spiked with 22.6 micrograms.ml-1 internal standard (podophyllic acid piperidinyl hydrazone nitroxide radical, GP-1) and extracted with ether--dichloromethane (3:1). The extract was evaporated at 45 degrees C. The residue was taken up with 0.1 ml of the mobile phase and 20 microliters aliquots were injected into the system. The calibration curve was linear in the range from 2 to 200 micrograms.ml-1 with r = 0.9997. The detection limit was 0.2 microgram.ml-1 and the recovery of GP-7 from rat plasma was 94.3%-100.9%. The relative standard deviations for within- day and between-day were 2.29%-4.64% and 5.55%-7.70%, respectively. After iv injection of GP-7 10, 20 and 30 mg.kg-1, the concentrations of the drug in rat plasma were determined. The pharmacokinetic parameters of GP-7 were obtained by using MCPKP program on a COMPAC-486 computer. The data obtained fitted a two-compartment open model, and the mean T1/2 beta value was 39.8 +/- 10.8 min.