1 Intradermal injection of prostaglandin (PG) D1 and D2 in the human forearm produced a long-lasting dose-related erythema. When compared with prostaglandin E1 or E2 the order of potency for erythema production was PGE1 greater than PGE2 greater than PGD2 greater than PGD1. 2 In rat skin, prostaglandin D2 but not D1 caused an increase in vascular permeability as quantitated by the Evans blue method and the 125I-albumin extravasation technique. Prostaglandin E2 was 3-5 times more potent than prostaglandin D2. 3 Prostaglandin D2 (10 ng) potentiated the increase in vascular permeability in rat skin produced by histamine, but not that produced by bradykinin. 4 Prostaglandin D2 (10, 20 and 50 ng) did not elicit oedema or hyperalgesia in the rat paw oedema test, but potentiated carrageenan-induced oedema; hyperalgesia was potentiated by doses of 100 ng and above.