Abstract.Hypericin, a photodynamic pigment, occurring in members of the Hypericaceae, can induce photosensitivity in grazing animals. The pigment has been isolated from the glandular trichomes located on the calyx ofHypericum hirsulum.Hypericin is shown to be capable of sensitizing the photo‐oxidation of methyl linolenate. This activity is reduced in the presence of crocin, a carotenoid. Evidence for the generation of singlet molecular oxygen by hypericin is provided by the monitoring of oxygen consumption during the photosensitized oxidation of imidazole. Rates of oxygen consumption were modified by deuterium oxide and sodium azide. The photodynamic action of hypericin on pea leaf discs results in the promotion of photo‐oxidative damage, measured by pigment loss and ethane production. These results are discussed in relation to the possible function of hypericin within the plant and the role of photo‐dynamic reactions in n