1 Clinically normal human abdominal skin was irradiated with three minimal-erythema doses of ultraviolet B irradiation, (u.v.B). 2 Erythema was assessed visually, and exudate recovered by a suction bulla technique from normal skin, and at 10 min, 2, 6, 18, 24 and 48 h after irradiation. 3 Erythema was barely visible at 2 h, but increased to maximum at 24 h, which was maintained at 48 h. 4 Increased arachidonic acid and prostaglandin E2, F2 alpha and D2 concentrations in the exudate, measured by gas chromatography-mass spectrometry, accompanied the developing erythema, with the maximal rise of arachidonic acid, prostaglandin E2 and D2 occurring at the height of the erythema at 24 h. 5 At 48 h, still at the height of the erythemal response, arachidonic acid and PGE2 levels had returned to near normal. 6 Concentrations of arachidonic acid and of its products from the cyclo-oxygenase pathway, parallel the development of i.u.B. erythema in the first 24 h.