All DHPs (nifedipine, nicardipine, nitrendipine) produced a concentration-dependent depression of the isometric contraction and of the atrial rate of the isolated, spontaneously beating atria of the guinea-pig. The depressive actions of nifedipine and nitrendipine were completely antagonized by the addition of calcium, aminophylline and isoprenaline. Aminophylline partially, calcium almost completely and isoprenaline completely antagonized the depressive action of nicardipine on the isometric contraction. Only isoprenaline antagonized the effect of DHPs on the atrial rate of the isolated, spontaneously beating atria of the guinea-pig. It is possible that all these substances restore the contractibility of the atria by compensating the calcium balance, previously changed by DHPs, or by producing an increase in the intracellular cyclic AMP content (aminophylline and isoprenaline).