Low-energy visible light (LEVL) has previously been found to modulate various processes in different biological systems. One explanation for the stimulatory effect of LEVL is light-induced reactive oxygen species formation. In the present study; both sperm and skin cells were illuminated with LEVL and were found to generate singlet oxygen (¹O2). The detection of ¹O2 was performed using a trapping probe, 2,2,6,6-tetramethyl-4-piperidone, coupled with electron paramagnetic resonance spectroscopy. In addition, we have shown that, together with ¹O2 generation, LEVL illumination increases the reductive capacity of the cells, which explains the difficulties encountered in ¹O2 detection. The potential of visible light to change the cellular redox state may explain the recently observed biostimulative effects exerted by LEVL. [ABSTRACT FROM AUTHOR]