The skin absorption of hypericin was evaluated in hairless mice to develop an optimised hypericin topical formulation that could be used in the clinical study of psoriasis. Hypericin (0.01-1.0%) in Beeler basis, polyethylene glycol ointment, carbopol gel, cetomacrogol cream, petrolatum or emulsifying ointment, with and without skin-absorption enhancers (isopropylidene glycerol and diethylene glycol monoethyl ether), was tested in-vivo on hairless mice skin. Using a skin-stripping technique and the intrinsic fluorescence of hypericin under standardised UV365 irradiation, it was demonstrated that the absorption of hypericin very much depended on the vehicle used. The concentrations of hypericin in the skin were then estimated by HPLC analysis. For this purpose, two vehicles were employed, with which hypericin penetrated the skin of hairless mice well (emulsifying ointment with isopropylidene glycerol) or very poorly (polyethylene glycol ointment). In the case of emulsifying ointment with isopropylidene glycerol (0.05% hypericin), a substantial concentration of hypericin (8.6+/-3.2 microg g(-1)) (mean +/- s.d., n = 5) was found in the skin. With polyethylene glycol ointment, however, only a limited hypericin skin concentration (0.38+/-0-34 microg g(-1), n = 5) was achieved. These results show that emulsifying ointment with polyethylene glycol holds promise as an effective topical vehicle for the treatment of skin diseases, such as psoriasis, with hypericin.