Guanine is determined at the 5.0×10−10 −2.0×10−7 mol/l level by differential-pulse adsorptive stripping voltammetry at a hanging mercury drop electrode using the reduction peak of its copper (II) complex at −0.21 V vs. Ag-AgCl electrode. The optimum analytical conditions were found to be Britton-Robinson buffer solution (pH 4.8), an accumulation potential of 0.0 V and an accumulation time of 3 min. Under these conditions, the detection limit is 5.0×10−10 mol/l and the relative standard deviation 2.6% for 1.0×10−7 mol/l guanine. The method is compared with the previous voltammetric methods. The presence of some purine derivatives does not interfere.