This master's thesis is focused on the determination of 6-thioguanine (6-T) by DC voltammetry (DCV), differential pulse voltammetry (DPV) and flow injection analysis with electrochemical detection (FIA-ED) on a boron doped diamond electrode (BDDFE). The optimum conditions for determination of 6-T were found and under these conditions, concentration dependences were measured and the limits of quantification (LOQ) were calculated for each method. Medium of 60% methanol and phosphate buffer (PB) pH 2,0 was chosen as optimum for DCV and DPV determination of 6-T at BDDFE. For both DCV and DPV, the linear concentration dependences were obtained in concentration ranges of 6-T from 2 µmol·l-1 to 10 µmol·l-1 with LOQ 0,9 µmol·l-1 for DCV and 1,5 µmol·l-1 for DPV. Lower LOQ was achieved in PB pH 2,0, where LOQ was 0,6 µmol·l-1 for both DCV and DPV, but the problem was lower repeatability. In PB pH 2 6-T was determined by DCV in drinking and river water with LOQ 1,3 µmol·l-1 in both drinking and river water. The possibility of solid phase extraction was investigated as a method for preliminary separation of 6-T from urine. For FIA under optimized conditions (polarization potential 1300 mV, flow rate 5 ml·min1 and sample volume 50 µl) the linear concentrations dependences were obtained in concentration ranges...