Human parathyroid hormone fragment, hPTH(1-34) and its analog, [Ala25,26,27]hPTH(1-34), were tested for their effects on rat blood pressure, tension generation in rat tail artery helical strips and intracellular calcium concentration ([Ca++]i) in smooth muscle cells of the rat tail artery. Amino acids at positions 25, 26 and 27 are reported to be responsible for the vascular action of the PTH molecule. The present studies demonstrate that hPTH(1-34), but not [Ala25,26,27]hPTH(1-34), produces a dose-dependent hypotensive effect in Sprague-Dawley (SD) rats and a dose-dependent relaxing effect on SD rat tail artery helical strips precontracted by norepinephrine, KCl or arginine vasopressin. Accordingly, hPTH(1-34), but not [Ala25,26,27]hPTH(1-34), inhibits the [Ca++]i increment induced by 15 mM KCl in cultured vascular smooth muscle cells (VSMC) from the SD rat tail artery after incubation with the peptide for 5 to 10 min. In addition, both hPTH(1-34) and [Ala25,26,27]hPTH(1-34) cause an initial transient and immediate increase in [Ca++]i in VSMC which does not correlate with any demonstrable vascular action. We suggest that PTH can elicit an initial increase in [Ca++]i and a subsequent inhibition of stimulated [Ca++]i increase in VSMC. The initial [Ca++]i effect is not related to its vascular action, whereas the subsequent inhibitory effect correlates well with its hypotensive effect.