This paper reports the results of a preliminaryin vivoinvestigation in which laser Doppler blood-flow measurements were made in a variety of tissues in the anaesthetized rat using a twin wavelength laser Doppler system. The aim was to assess whether there was any detectable difference in the depth response of the laser Doppler system to blood-flow in skin, muscle and brain tissue using red light (633 nm) from a He—Ne laser source and near infrared (NIR) light (780 nm) from a semiconductor diode laser source. The results show that under normalized conditions, the flow signals obtained from the near infrared laser Doppler system is consistently larger than that recorded from the red He—Ne laser Doppler system. This suggests that laser Doppler flowmetry systems based on NIR laser sources sample larger volumes of tissue than those based on red He—Ne lasers.