No current method of flap monitoring is ideal for use in all types of free tissue transfers. No method provides objective, easily communicated data that is identical in all types of transfers. In particular, reliable monitoring of buried transfers has proved difficult with available methods. The rat anterior thigh flap based on the external iliac vascular pedicle was introduced by us as a model of deep free tissue transfer. Four sets of 10 flaps were raised in the following groups: Group A (control), Group B (arterial occlusion), Group C (venous occlusion), and Group D (arterial and venous occlusion). Postoperative muscle flap pH was measured with a micro-pH electrode (1.2 mm) and correlated with arterial blood gas. Results showed excellent correlation of flap and serum pH over time (mean flap pH, 7.28; mean serum pH, 7.30). Arterial occlusion produced a rapid drop in flap pH of 0.66 pH units at 1 hour. Venous occlusion pH drop was 0.27 pH units at 1 hour, 0.53 pH units at 3 hours. Arterial and venous occlusion produced a pH drop of 0.55 pH units at 1 hour. The most rapid rate of pH drop occurred immediately after vessel occlusion. We have used continuous pH monitoring in 21 free tissue transfers for up to 84 hours after surgery. PH values remained constant in each transfer (range, 7.20–7.50; grand mean, 7.35). There was one flap failure among the monitored group of flaps, which was predicted by pH drop before loss of Doppler pulse. We provide experimental and clinical documentation that continuous pH monitoring can provide objective data regarding the viability of free tissue transfers with constant, identical values in all types of free tissue transfers, including buried transfers.