Snarf-1-AM (Seminaphthorhodafluor-1-acetoxymethylester) being used as a fluorescent pH probe, confocal laser microspectrofluorometric measurements allowed intracellular pH to be assessed within 3T3 mouse fibroblasts in the presence or in the absence of hypericin. Cytoplasmic local pH drops of 0.4 unit have been measured in the presence of 1 mu M hypericin after 90 s of exposure to 0.1 mu W of illumination at 514 nm. Moreover, time course experiments clearly show that this local pH decrease is actually light dose dependent. Taking into account previously reported results and interpretations, it is shown here that this pH decrease has to be considered as one of the possible processes responsible for the photosensitizing properties of hypericin, involved in its virucidal and antitumor activities.