The method of laser Doppler (LD) fluxmetry records the flux of moving blood cells and particles in a small tissue volume. However, also during complete arterial occlusion a flux-independent signal can be recorded, which is sometimes significantly different from the electrical zero of the instrument. This part of the signal has been called the biological zero (BZ), and can sometimes represent up to 80% of the total LD signal. In the present study, we investigated the influence of induced hyperemia produced by methylnicotinate and histamine respectively, both vasodilators but only histamine giving edema. Eleven legs of nine healthy subjects were studied. Test sites on the dorsal skin of the foot were prepared with a) histamine (10 mg/ml), b) methylnicotinate (0.1 mol/l) and c) saline. The substances were administered onto the skin with a lancet as the prick test procedure. The basal LD was measured before, and the BZ during an arterial occlusion obtained with a cuff at the ankle. Histamine induced a wheal and edema in all subjects, while methylnicotinate only caused erythema. Basal LD was significantly (p less than 0.01) increased by both substances compared to saline. The BZ was significantly higher (p less than 0.01) with histamine than with both saline and methylnicotinate. The results show that BZ is increased by vasodilation, and still more so by edema formation. It is concluded that the BZ value must always be recorded and taken into consideration when LD blood cell flux is measured in the skin.