1. Aminophylline inhibited the contractions induced in rat isolated aorta by high K and noradrenaline (NA) as well as the transient contraction induced by NA in Ca-free media and suppressed the spontaneous myogenic activity in portal vein segments. 2. Aminophylline had no significant effects on 45Ca influx in resting aorta but inhibited 45Ca influx stimulated by high K or NA. 3. It is concluded that aminophylline inhibited Ca entry through voltage- and receptor-operated channels and NA-induced Ca release from intracellular stores. A part of the vascular inhibitory effects of aminophylline may be mediated by a cAMP-dependent mechanism.