The present study was undertaken to investigate the effects of extracellular pH (pHe) and intracellular pH (pHi) on 5-hydroxytryptamine (5-HT)-induced contraction and Ca2+ mobilization in vascular smooth muscles. Strip preparations of the rabbit basilar artery without endothelium were loaded with 40 microM fura-2-AM and 2 microM BCECF-AM and mounted in an organ bath. The isometric tension was recorded by using a force displacement transducer. Administration of 5-HT caused dose-dependent contraction in the rabbit basilar arteries. Acidification of pHe from 7.40 to 6.90 reduced the 5-HT-induced contraction and [Ca2+]i transients. Alkalinization of pHe from 7.40 to 7.90, on the other hand, enhanced the contraction and elevation of [Ca2+]i. In the other series of experiments, pHi (7.12 in normal PSS) was selectively altered by adding either butyric acid or trimethylamine. Intracellular acidification (pHi = 6.89) and alkalinization (pHi = 7.35) without changes in pHe produced qualitatively similar effects to those caused by extracellular acidification and alkalinization, respectively. Ca-sensitivity, which is defined as Deltatension/Delta[Ca2+]i, was not affected by the alteration of pHe nor pHi. In the Ca2+-free solution, the addition of 5-HT produced transient increases in [Ca2+]i and isometric tension that were much smaller than those in the normal physiological salt solution. The 5-HT-induced responses of [Ca2+]i and tension in the Ca2+-free solution were not affected by acidification nor alkalinization. These results suggest that a 5-HT-induced contraction is significantly modulated by pH through changing the [Ca2+]i transients, and that the change of pHi plays, at least in part, a role in the alteration of 5-HT-induced contraction resulting from acidosis or alkalosis in the rabbit basilar artery.