A sampled dc polarogram of a reversible redox reaction complicated by the reactant adsorption on the surface of the static mercury drop electrode may consist of a main wave and a post-wave which are separated by a minimum. This phenomenon is analysed theoretically for Langmuir and Frumkin isotherms. The minimum is a part of the postwave. It appears in the potential range in which the maximum surface coverage is diminished below 0.8. The minimum is the consequence of the change of the current-time relationship during the life-time of a single drop. A minumum followed by a very steep postwave is an indication of strong lateral attractions in the adsorbed layer.