With a new fluorescence probe of Ca2+, STDIn-AM, 5-hydroxytryptamino (5-HT)-induced spontaneous calcium release events (calcium sparks) in cultured rat stomach fundus smooth mus-cle cells (SFSMC) are investigated by laser scanning confocal microscope. The mechanisms of initiation of Ca2+ sparks, propagating Ca2+ waves and their relation to E-C coupling are discussed. After the extracellular [Ca2+] is increased to 10 mmol/L, addition of 5-HT causes hot spots throughout the cytoplasm, which is brighter near the plasmalemma. The amplitude of the event is at least two times greater than the standard deviation of fluorescence intensity fluctuations meas-ured in the neighboring region and the duration of the Ca2+ signal is over 100 ms. The results sug-gest that 5-HT acts by the way of 5-HT2 receptors on SFSMC, then through 5-HT2 receptors cou-ples IP3/Ca2+ and DG/PKC double signal transduction pathways to cause Ca2+ release from intra-cellular Ca2+ stores and followed Ca2+ influx possibly through calcium release-activated calcium influx. The acceptor of activated 5-HT2 can also cause membrane depolarization, which then stimulates the L-type Ca2+ channels leading to Ca2+ influx. Then the local Ca2+ entry mentioned above activates ryanodine-sensitive Ca2+ release channels (RyR) on sarcoplasmic reticulum (SR) to cause local Ca2+ release events (Ca2+ sparks) through calcium-induced calcium release (CICR). With a new fluorescence probe of Ca2 +, STDIn-AM, 5-hydroxytryptamino (5-HT) -induced spontaneous calcium release events (calcium sparks) in cultured rat stomach fundus smooth muscle cells (SFSMC) were investigated by laser scanning confocal microscope . The mechanisms of initiation of Ca2 + sparks, propagating Ca2 + waves and their relation to EC coupling are discussed. After the extracellular [Ca2 +] was increased to 10 mmol / L, addition of 5-HT causes hot spots throughout the cytoplasm, which is brighter The amplitude of the event is at least two times greater than the standard deviation of fluorescence intensity fluctuations meas-ured in the neighboring region and the duration of the Ca 2+ signal is more than 100 ms. The results sug-gest that 5- HT acts by the way of 5-HT2 receptors on SFSMC, then through 5-HT2 receptors cou-ples IP3 / Ca2 + and DG / PKC double signal transduction pathways to cause Ca2 + release from intra-cellular Ca2 stores and subsequently Ca2 + influx possibly th The acceptor of activated 5-HT2 can also cause membrane depolarization, which then stimulates the L-type Ca2 + channels leading to Ca2 + influx. Then the local Ca2 + entry above above activates ryanodine-sensitive Ca2 + release channels ( RyR) on sarcoplasmic reticulum (SR) to cause local Ca2 + release events (Ca2 + sparks) through calcium-induced calcium release (CICR).