A method of cyclic voltammetry with adsorptive preconcentration is proposed, exploiting the differences in the behaviour of open circular or linear DNA and closed circular DNA under denaturing conditions. The height of the anodic peak of single-stranded DNA is inversely proportional to the content of closed circular DNA in the sample. The sensitivity and concentration range of the determination depends on the time of adsorptive preconcentration used. A linear dependence of the height of the anodic peak on the content of the DNA of the denatured plasmid ColE1 was observed in the concentration range 0.25 to 0.2 μg DNA/ml with 4 min adsorptive preconcentration. With double this time one could detect down to 0.05 μg DNA/ml. The determination of single-stranded DNA was unaffected by the presence of an excess of duplex DNA in the sample analysed. The method is fast and requires less than a microgram of plasmid DNA.