5-aminolaevulinic acid (5-ALA) is clinically used during photodynamic therapy to induce apoptosis in cancer cells. In a recent study we could demonstrate the effects of 5-ALA on mechanical activity. The cellular mechanisms however are not fully understood. Using microfluorescence, Fura- 2 loaded tissues from urinary bladder and gastric fundus (rat), contraction as well as intracellular calcium were simultaneously measured. Using an automated apparatus with two shutters - to illuminate the tissue or to open the optical path to the photomultiplier - a chopped exposure sequence was performed. Phases of illumination were 20s, phases of detection were 2s. Having a sampling rate of 50/s the mean values of intracellular calcium (quotient) and of tension were calculated. During such procedure the intracellular calcium and mechanical activity increased rhythmically. Interestingly, the frequency of the observed rhythm was not the same as the illumination rhythm. It is concluded, that intracellular calcium accumulates in smooth muscle during treatment with 5-ALA and illumination which may be an important step for the tissues to lose function. The dynamic behaviour may be due to both: calcium pump activity and the frequency of illumination cycles.