Change of intracellular free Ca in smooth muscle was monitored using Quin 2, Ca sensitive fluorescence dye. Upon electrical stimulus, increase of light emission from the sample occurred in two phases. Procaine and caffeine inhibited and facilitated, respectively, the 2nd phase of intensity change, which coincided with the mechanical response. It is concluded that the released Ca from intracellular store sites may play a key role for the mechanical activation of smooth muscles.