Amethopterine, a folic acid antagonist extensively used as an anti-neoplastic agent, adsorbs strongly on to mercury electrodes. Similar behaviour is also shown by other pterines such as folic and folinic acids. By using this phenomenon to accumulate these compounds at a static mercury drop electrode, prior to differential-pulse voltammetric measurements, very high sensitivities can be readily achieved. The influence of several variables (such as the accumulation time, accumulation potential, analyte concentration and the presence of surfactants) on the adsorptive stripping response has been evaluated. Applications to serum drug determination are also presented.