Reliable early detection of adverse circulatory changes within a flap following free tissue transfer and early re-exploration are vital to minimise flap failure. Most surgeons rely on clinical assessment to monitor these changes but techniques such as plethysmography and laser Doppler have their advocates. These methods are limited however to measuring changes close to the surface. Near infra-red spectroscopy (NIRS) is a relatively new, non-invasive technique which allows continuous monitoring of concentration changes in oxy-, deoxy- and total haemoglobin (HbO2, Hb and HbT), as well as oxidised cytochrome aa3, through tissue up to 10 cm in depth. Information is provided on tissue oxygen supply, cellular oxygen utilisation, blood volume and perfusion status. A study has been performed in 10 rabbit hind limbs to assess the ability of NIRS to detect and distinguish between venous, arterial and total vascular occlusion. Clear patterns of change have been identified which allow rapid detection of vascular occlusion with accurate prediction of site. Arterial occlusions were characterised by an increase in Hb with a corresponding decrease in HbO2 and HbT. Venous occlusions resulted in an increase in HbT with relatively minor fluctuations in Hb and HbO2. Simultaneous occlusion of both artery and vein produced similar changes to those of arterial occlusion except that HbT decreased only minimally. These findings suggest that NIRS has a potentially useful role in the monitoring of free flaps, with the great advantage that perfusion can be measured to a considerable depth and information provided on the oxygenation profiles both accurately and non-invasively.