The effects of verapamil (V) on the contraction of rabbit aortic strips induced by KCl and rioradrenaline (NA) were studied. In normal Krebs' solution, the contractions induced by NA were more intensively inhibited by V than those induced by KCl. After the preparations had been exposed for 2 hours to calcium-free solution, the contractile responses to NA decreased more markedly than those to KCl. When V was added to the calcium-free solution, the NA-induced responses were further inhibited, but the KCl-indu...