Objective: The primary goal of this work was to determine the relative importance of sarcoplasmic reticulum inositol 1,4,5-triphosphate receptors and ryanodine receptors in the mechanism of intracellular calcium wave propagation in human uterine myocytes. A secondary goal was to identify the rate-determining step of calcium wave propagation. Study Design: Pregnant human myometrium was obtained at the time of cesarean delivery, enzymatically dispersed, and cultured through several passages. Intracellular calcium wave velocities were measured with video fluorescence microscopy and the calcium-dependent fluorescent dye calcium green 1. Experimental conditions were modified by exposure of the cells to ruthenium red (blocked ryanodine receptor), ryanodine (locked open ryanodine receptor), oxytocin (increased inositol-1,4,5-triphosphate), sodium butyrate (intracellular acidification), ammonium chloride (intracellular alkalinization), and elevation of temperature (from 19°C to 30°C). Results: Wave velocities were found to be the same for spontaneously occurring (9.6 ± 2.6 μm/s) and oxytocin-stimulated (10.3 ± 3.4 μm/s) waves. Advance treatment of the cells with ryanodine or ruthenium red failed to change oxytocin-stimulated wave velocities from control values. The temperature dependence of calcium wave velocities was studied across the range 19°C to 30°C. Plots of wave velocities versus the inverse of the temperature yielded apparent activation energies that were the same for spontaneous (13.2 ± 0.3 kcal/mol) and oxytocin-induced (14.3 ± 1.6 kcal/mol) waves. After intracellular acidification by treatment with butyrate (20 mmol/L) wave velocities increased by 44%. Wave velocities decreased by 35% after treatment with ammonium chloride (20 mmol/L). Conclusion: Propagation of intracellular calcium waves in cultured human uterine myocytes exhibited mechanisms of sarcoplasmic reticulum calcium release that could use either inositol 1,4,5-triphosphate receptors alone or ryanodine receptors alone, or both together. The rate-determining step for calcium wave propagation was diffusion of calcium though a highly buffered cytoplasm. (Am J Obstet Gynecol 2001;184:1228-34.)