Summary—The effects of the dihydropyridine calcium antagonist nicardipine on the concentration‐response curves of relaxant compounds acting through the adenylate‐cyclase/cAMP system (isoprenaline, forskolin, adenosine and theophylline) or by the cGMP pathway (sodium nitroprusside) were studied on human isolated bronchus and guinea‐pig isolated trachea. These effects were compared with those of nifedipine (a dihydropyridine derivative) and theophylline (a non‐selective phosphodiesterase inhibitor). Nicardipine, in the range of 0.01 to 1 μM, significantly potentiated the relaxant effects of isoprenaline, forskolin, adenosine and theophylline, whereas the effects of sodium nitroprusside were significantly potentiated at 10 μM only. These results suggest that nicardipine behaves as an inhibitor of phosphodiesterases III and IV. One such effect may be involved in the potentiation of the isoprenaline relaxation of human and guinea‐pig iso