Dermal blood flow was evaluated after skin prick test with histamine and allergen in six patients with seasonal allergic rhinitis. Blood flow was registered continuously for 60 minutes after the test procedure with laser doppler flowmetry, which allows noninvasive measurements. Blood flow was measured close to the skin test in the wheal obtained, and at a greater distance from the prick in the flare reaction. Tests were performed with preloaded skin test needles with histamine and the appropriate allergen freeze-dried on the point of the lancet, as well as with the appropriate negative control. The prick test procedure, by itself, induced a transient increase in blood flow that was normalized again after 9 minutes for the closest measurement. Histamine induced a rapid increase in blood flow in both the flare and wheal reaction that was normalized after about 45 minutes. The increase was significantly higher in the flare compared to the wheal for the time points from 6 1/2 to 13 minutes. Allergen induced a similar increase in blood flow. However, the increase was not noticeable until 2 1/2 minutes after the allergen application and was not completely abolished within 60 minutes. Furthermore, the difference between the flare and wheal reaction, with the higher values for the flare reaction, was present for a longer period of time than for the equivalent histamine measurements. In conclusion, laser doppler flowmetry appears useful for continuous evaluation of vascular changes induced at skin prick tests.(ABSTRACT TRUNCATED AT 250 WORDS)