The purpose of this study was to elucidate tissue changes occurring within an ischaemic flap by monitoring the blood biochemistry, and to evaluate these changes in relation to ultimate flap viability. A rabbit epigastric free flap was made ischaemic for 4 days at 6 degrees C, then revascularized by anastomosis of its femoral artery and vein. An identical free flap immediately revascularized in another group of rabbits served as a control. The viability of the free flap, as well as various biochemical parameters studied by drawing blood from a catheter in the ear vein, were observed daily. Immediately after the revascularization of ischaemic flaps, there was a 16-fold increase in the plasma levels of creatine kinase (CK) and a smaller but significant 1.5-fold to 2.0-fold increase in lactate dehydrogenase (LDH) and aspartate aminotransferase (AST). In flaps which ultimately failed by 7 days post-ischaemia, the plasma levels of CK, LDH and AST peaked at day 2 post-ischaemia at 68, 13 and 8 times normal respectively, whereas in flaps which survived, the levels of these enzymes recovered to normal by day 3 post-ischaemia. These enzymic changes are probably due to a combination of ischaemic changes in the flap vasculature, ischaemic changes in the flap muscle, and inflammatory changes in the surrounding abdominal tissue. The plasma levels of CK at any time post-ischaemia, and particularly in the first 24 h, were significantly higher in ischaemic flaps which failed compared with those which survived. This parameter is therefore proposed as a possible means of predicting potential flap failure after ischaemic insult, in time to make appropriate surgical intervention.