A new fluorescent Ca2+ indicator STDIn-AM for detecting [Ca2+]i transients in cultured smooth muscle cells is presented. By making a comparison, the difference between STDIn and fluo-3 is discussed in detail. Using the new Ca2+ indicator, the mechanism of 5-hydroxytryptamino (5-HT) induced intracellular calcium dynamics in stomach fundus smooth muscle cells (SFSMC) of rats is investigated. It is shown that in contrast with fluo-3, STDIn is uniformly distributed in the cytosolic compartment but excluded from the nucleus, when it is transfected into cells. This feature makes it a real cytosol Ca2+ indicator and can reflect changes of cytosol [Ca2+] more accurately than that of fluo-3. In addition, STDIn responds to the [Ca2+]i transients more sensitive and faster than fluo-3. The results also show that, the L-type Ca2+channel inhibitor Mn9202 and the PLC inhibitor Compound 48/80 can significantly inhibit the [Ca2+]i elevation induced by 5-HT, while the PKC inhibitor D-Sphingosine can enhance the effect of A new fluorescent Ca2 + indicator STDIn-AM for detecting [Ca2 +] i transients in cultured smooth muscle cells is presented. By making a comparison, the difference between STDIn and fluo-3 is discussed in detail. Using the new Ca2 + indicator, the mechanism of 5-hydroxytryptamino (5-HT) induced intracellular calcium dynamics in stomach fundus smooth muscle cells (SFSMC) of rats is investigated. It shows shown in contrast with fluo-3, STDIn is uniformly distributed in the cytosolic compartment but excluded from the nucleus , when it is transfected into cells. This feature makes it a real cytosol Ca2 + indicator and can reflect changes of cytosol [Ca2 +] more accurately than that of fluo-3. In addition, STD In responds to the [Ca2 +] i transients more sensitive and faster than fluo-3. The results also show that the L-type Ca2 + channel inhibitor Mn9202 and the PLC inhibitor Compound 48/80 could significantly inhibit the [Ca2 +] i elevation induced by 5-HT, while the PKC inhibitor D- Sphingosine can enh ance the effect of