Objective  To observe the spontaneous Ca

2+

transients of smooth muscle cells (SMCs) and explore its mechanism in the rat bladder. Methods  Single layer of smooth muscle bundles attached to the serosal layer were prepared and superfused with warmed physiological saline to recover and maintain their activity. After spontaneous muscle movements of the tissues were visually observed

the muscle layers were loaded with

the fluo-4 and then spontaneous Ca

2+

transients of SMCs in the muscle layers were observed by confocal laser scanning microscopy. Nimodipine

2-aminoethoxydiphenylborate (2-APB)

ryanodine and thapsigargin were then used to investigate the mechanism of spontaneous Ca

2+

transients of SMCs. Results  Spontaneous Ca

2+

transients of SMCs in the smooth layers were successfully detected. Amplitude (F/F0) and frequency of spontaneous Ca2+ transients of SMCs in the smooth layers were（2.20±0.13）and（10.31±2.74）times/min. Spontaneous Ca

2+

transients of SMCs were completely abolished by nimodipine. Amplitude and frequency of spontaneous Ca

2+

transients were significantly reduced to （1.98±0.14） and （8.18±1.97）times/min respectively by 2-APB (P0.05

n=18). Similarly

amplitude and frequency of spontaneous Ca2+ transients of SMCs were significantly decreased to（1.81±0.11）and（7.59±2.16）times/min respectively by ryanodine (P0.01

n=17). Amplitude of spontaneous Ca

2+

transients were notably suppressed to（1.76±0.11）(P0.01

n=20) by thapsigargin.  Conclusion  SMCs generate spontaneous Ca

2+

transients via extracellular Ca

2+

entry through L-type Ca

2+

channels

which may be supported by Ca

2+

release from intracellular Ca

2+

stores.