The clinical use of Ca++ antagonist agents as modulators of multidrug resistance is limited by their strong vasodilator activity. This study reports data obtained by testing a series of new 1,4 dihydropyridine derivatives (DHPs) for their in vitro resistance modulating activity and their Ca++ antagonist effect. All the tested DHPs are active to increase doxorubicin activity with dose modifying factor values ranging between 2 and 47 on P388/DX cells and 12 and 36 on LoVo/DX cells. Their resistance modulating action is exerted through an increase of DX intracellular level. The Ca++ antagonist activity of DHPs, evaluated as capacity to inhibit the KCl-induced contractions in isolated Guinea pig ileum strips, is not related to their resistance modulating activity. This finding makes it possible to select, for further in vivo evaluations, compounds IX, X and XI, which have strong ability to overcome multidrug resistance and low Ca++ antagonist effect.