n vascular smooth muscle cells, Ca2+ release via IP(3) receptors (IP(3)R) and ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR) Ca2+ store contributes significantly to the regulation of cellular events such as gene regulation, growth and contraction. Ca2+ release from various regions of a structurally compartmentalized SR, it is proposed, may selectively activate different cellular functions. Multiple SR compartments with various receptor arrangements are proposed also to exist at different stages of smooth muscle development and in proliferative vascular diseases such as atherosclerosis. The conclusions on SR organization have been derived largely from the outcome of functional studies. This study addresses whether the SR Ca2+ store is a single continuous interconnected network or multiple separate Ca2+ pools in single vascular myocytes. To do this, the consequences of depletion of the SR in small restricted regions on the Ca2+ available throughout the store was examined using localized photolysis of caged-IP3 and focal application of ryanodine in guinea-pig voltage-clamped single portal vein myocytes. From one small site on the cell, the entire SR could be depleted via either RyR or IP(3)R. The entire SR could also be refilled from one small site on the cell. The results suggest a single luminally continuous SR exists. However, the opening of IP(3)R and RyR was regulated by the Ca2+ concentration within the SR (luminal [Ca2+]). As the luminal [Ca2+] declines, the opening of the receptors decline and stop, and there may appear to be stores with either only RyR or only IP(3)R. The SR Ca2+ store is a single luminally continuous entity which contains both IP(3)R and RyR and within which Ca2+ is accessed freely by each receptor. While the SR is a single continuous entity, regulation of IP3R and RyR by luminal [Ca2+] explains the appearance of multiple stores in some functional studies.