The scattering and absorption of light by tissue and blood is wavelength dependent; the tissue penetration of green light (lambda = 543.5 nm) is about 60 per cent of that of red light (lambda = 632.8 nm) but the absorption of green light by blood is about 20 times greater than for red light. The effect of this difference has been studied by observing the responses of skin blood flow to heat and weal, measured by laser Doppler flowmetry at the two wavelengths. By using time autocorrelation function analysis (ACF) of the scattered light measured, low and high frequency components have been associated with capillary and larger vessel flow, respectively. The comparison of ACF from scattered green and red light has shown that measurements cannot be interpreted by only considering light penetration depth through a homogeneous tissue. Light absorption and multiple scattering by blood at the individual microvessel level, blood rheology and vessel morphology are parameters which are considered for greater attention.