The tcPO2 electrode used at 37 degrees C monitors changes in cutaneous PO2 which at this temperature is mainly determined by the changes in blood flow. Laser Doppler velocimetry (LDV) measures the product of red cell number times their velocity. A comparison of these methods for the detection of skin reactive hyperemia has been made with a combined tcPO2 and laser Doppler sensor. The two parameters increased and decreased in parallel, the tcPO2 signal being delayed 10-20 s compared to the LDV signal. A significant correlation between the two signals was obtained (r = 0.73-0.93). In contrast, no significant correlation existed between the postocclusive peaks of the two signals recorded in 12 duplicate experiments. Comparing the peak amplitudes from the repeated recordings the tcPO2 signal showed a higher correlation coefficient (r = 0.91) and smaller intercept than the LDV signal (r = 0.74). Both methods can be used for non-invasive recordings of skin reactive hyperemia. The tcPO2 signal at 37 degrees C reflects changes in blood flow of the most superficial capillaries and reactive hyperemia can be monitored with high reproducibility. Only this signal can be used for calculation of skin blood flow. The LDV signal conveys information of blood flow changes mainly of deeper dermal vascular beds and reacts virtually instantaneously, but is less reproducible.