The intestinal absorption characteristics of protohypericin, a protonaphthodianthrone present in Hypericum extract, were studied and compared with those of hypericin. The Caco-2 model was used as a model of the intestinal mucosa to assess transepithelial transport and cell uptake. The experimental work was performed in specific light conditions that prevented both the photoconversion of protohypericin into hypericin and the photosensitization of the cells. Following application of the individual compounds (80-200 microM) to the apical side of the monolayers, the appearance in the basolateral compartment was found to be very low (<0.5%/5 h), but was comparable for both compounds. A lag-time of 2-3 h was observed, suggesting gradual saturation of binding sites on the membrane or inside the cells. Uptake experiments of protohypericin and hypericin by Caco-2 cells revealed a very significant cellular accumulation (4-8%); uptake was characterised by saturation after 3 h. The findings of this study suggest that protohypericin has comparable absorption characteristics as hypericin and may contribute to the beneficial effect of Hypericum extract after oral dosing.