Histamine-induced changes in blood flow in normal human skin were assessed by laser-Doppler velocimetry using 2 lasers of different wavelengths: 780 nm infrared and 633 nm helium-neon. The visible flare response in skin was also measured by planimetry. Laser-Doppler velocimetry was shown to detect both the magnitude of the change in blood flow caused by histamine and also the extent of the changes in the skin. Both parameters were related to the dose of histamine, which ranged between 25 and 750 pmol/site. There was good correlation between the magnitude of the histamine-induced blood flow change and the extent of the response. The flare induced by histamine and measured by planimetry was similar in extent to the blood flow change recorded by laser-Doppler velocimetry. No difference in either the magnitude or the extent of blood flow changes in response to histamine, as measured by laser-Doppler velocimetry, were found when lasers of different wavelengths were used.