||The effect of leukotriene D4 on the nasal mucosal blood flow in humans was measured using a laser doppler flow meter. The leukotriene solution was applied topically to the nasal mucosa in 9 healthy subjects, and changes in blood flow were measured non-traumatically by the laser doppler instrument. The response showed a consistent increase in blood flow. A dose-response was found in the range of 0.063 – 4.0 nanomole. These results confirm an earlier study on the human skin, implicating leukotriene D4 as an important vasodilator in humans. No increase in nasal secretion was noted by the subjects tested.