1. These studies describe the functional effects of modulation of the sarcoplasmic reticulum (SR) Ca2+ stores at three levels of the vasculature: (i) large arteries (rat and guinea-pig aorta); (ii) small resistance arteries (rat tail artery, rabbit mesenteric artery, dog mesenteric artery); and (iii) arterioles (guinea-pig submucosal arterioles of the small intestine). 2. All tissues responded to phenylephrine (PE; 10 mumol/L) with a transient contraction in Ca(2+)-free Krebs', reflecting Ca2+ release from PE-sensitive Ca2+ stores. After pretreatment with cyclopiazonic acid (CPA; 30 mumol/L) or thapsigargin (TSG; 1 mumol/L), putative SR Ca2+ pump inhibitors, the PE-induced contraction in a Ca(2+)-free medium was significantly inhibited in arterial tissues at all levels of the vasculature. Similarly, ryanodine (RYA; 30 mumol/L), an agonist that enhances Ca2+ release from the SR, also reduced the PE contraction in a Ca(2+)-free solution. 3. CPA or TSG alone in the presence of extracellular Ca2+, caused marked and sustained contraction in the rat and guinea-pig aorta and marked but transient or no contraction in the resistance arteries. In the rat and guinea-pig aorta, RYA caused a slowly developing tension. Little increase in basal tension was produced by RYA in resistance arteries and arterioles. 4. The findings show that an agonist-releasable Ca2+ pool is present at all levels of the vasculature that is independent of the size of the vessels and suggest that under normal physiological conditions there is an intimate balance between the roles of the plasma membrane and of the SR in the maintenance of vascular contractility. It appears that the role of the SR diminishes as the arteries become smaller, while Ca2+ fluxes across the plasma membrane predominates.