The local effects of intracutaneous injections into humans of 1-3 nmol of five products of arachidonic acid metabolism, leukotrienes (LT) C4, D4, E4, and B4 from the 5-lipoxygenase pathways and prostaglandin (PG) D2 from the cyclooxygenase pathway, were assessed clinically and histologically. In equimolar concentrations, LTC4, LTD4, and LTE4, elicited erythema and wheal formation, in which a wheal with central pallor was present up to 2 hr, and the erythema persisted as long as 6 hr. PGD2 elicited a wheal that lasted up to 1 hr and erythema that lasted up to 2 hr. The dermal vascular sites affected by LTD4 and PGD2 included capillaries, superficial and deep venules, and arterioles. LTB4 elicited a transient wheal and flare, followed in 3-4 hr by induration that was characterized by a dermal infiltrate comprised predominantly of neutrophils. The combination of LTB4 and PGD2 elicited tenderness and increased induration associated with a more intense neutrophil infiltration. Thus, the products of the 5-lipoxygenase pathway of arachidonic acid metabolism in nanomole amounts can induce cutaneous vasodilation with edema formation and a neutrophil infiltrate, and these responses are enhanced by a cyclooxygenase pathway product, PGD2.