Using a laser-Doppler-flowmeter the microvascular response to LTC4 and LTD4 was measured. Intradermal injection of 1 microgram LTC4 and LTD4 caused an increase in the microvascular cutaneous bloodflow. The increase in flow was equal to that caused by histamine in equimolar amounts. Blocking the triple-response did not change the response. The values measured after injection of histamine and leucotrienes were about 10-15 times the values found in undisturbed skin and represents probably a maximally dilated vascular bed. Injection of the leucotrienes caused a slight sensation of pain.