The effects of sodium nitroprusside (NaNP) and diltiazem on CaCl2-,KCl- and histamine-induced contractile responses were studied in isolated rabbit basilar artery, aorta, taenia coli and trachea. NaNP reduced CaCl2-induced maximal contractions in basilar artery and aorta, but little affected those in taenia coli and trachea. NaNP reduced histamine-induced maximal contraction markedly in aorta but slightly in basilar artery and taenia coli. The reduction in aorta was reversed by increasing CaCl2 contraction in bathing medium. NaNP inhibited histamine-induced contractions of basilar artery, aorta and taenia coli in Ca-free medium. Diltiazem reduced CaCl2-induced maximal contractions in basilar artery and aorta, whilst it caused a parallel shift of concentration-response curves for CaCl2 to the right in taenia coli and trachea. diltiazem reduced histamine-induced maximal contractions in basilar artery and taenia coli. These reductions were reversed by increasing CaCl2 concentration in bathing medium. Diltiazem strongly inhibited histamine-induced contraction of taenia coli in Ca-free medium. The effects of NaNP and diltiazem on KCl-induced contractions both in normal (Ca; 2.5 mM) and in high Ca (Ca; 12.5 mM) media were also studied. From the results obtained in this study, the actions of NaNP and diltiazem were considered to vary with the type of smooth muscle or the condition eliciting contraction. Their possible mechanisms were discussed in this paper.