||Leukotriene D4 (LTD4) increased the blood flow rate in human skin, equipotent to histamine in the dose range of 3.1-200 pmol. The vasodilatation lasted for up to 60 min, and no late reactions occurred. Indomethacin did not affect the LTD4-induced blood flow rate. H1 and H2 antagonists reduced the increase in blood flow rate, but did not abolish the response to LTD4. Local nerve block inhibited the axon reflex-mediated flare component of the LTD4-induced blood flow rate, leaving a local red reaction. This local red reaction was not affected by H1 and H2 antagonists. These results indicate histamine as a mediator of the axon reflex, and show that LTD4 causes a direct vasodilatory effect that is not mediated via histamine or cyclooxygenase products. The laser-Doppler flowmeter was applied for dynamic studies of the vasopressor response in the skin during a Valsalva maneuver, and the relative changes in blood flow were confirmed by control estimates of the blood flow rate by a 133xenon washout method. The pressor response to a Valsalva maneuver was reversed by local nerve block, but not affected by LTD4. Therefore LTD4 did not interfere with the sympathetic activity on the cutaneous vessels. Leukotriene D4 caused a dose-dependent wheal reaction, equipotent to histamine in the dose range of 0.2-200 pmol. Only minor whealing occurred when the vasculature to the test arm was occluded before injection of LTD4 and the circulation restored 30 min later. Most of the LTD4 was apparently metabolized within this period. Subsequent injections of LTD4 into the same sites demonstrated the development of tachyphylaxis with respect to whealing. This evidence suggests that LTD4 cannot mediate sustained inflammation. The injections of LTD4 caused neither pain nor itching. In conclusion, the elucidated properties point to LTD4 as a possible mediator of microvascular changes during acute inflammation.