The photodynamically active plant pigment hypericin, a characteristic metabolite of Hypericum perforatum (St. John's wort), is widely used as an antidepressant. When administered orally, phototoxic symptoms may limit the therapeutic use of hypericin-containing drugs. Here we describe the high-performance liquid chromatographic (HPLC) detection of hypericin and semiquantitative detection of pseudohypericin in human serum and skin blister fluid after oral single-dose (1 x 6 tablets) or steady-state (3 x 1 tablet/day, for 7 days) administration of the Hypericum extract LI 160 in healthy volunteers (n = 12). Serum levels of hypericin and pseudohypericin were always significantly higher than skin levels (p </= 0.01). After oral single-dose administration of Hypericum extract the mean serum level of total hypericin (hypericin + pseudohypericin) was 43 ng/ml and the mean skin blister fluid level was 5.3 ng/ml. After steady-state administration the mean serum level of total hypericin was 12.5 ng/ml and the mean skin blister fluid level was 2.8 ng/ml. These skin levels are far below hypericin skin levels that are estimated to be phototoxic (>100 ng/ml).