Changes in myocardial membrane biochemistry and ultrastructure, determined shortly (2 h) after reperfusion of ischaemic myocardium, were compared with the long term (4 wk) recovery of regional myocardial function. Anaesthetised pigs were subjected to 30 min (n = 14, group I) or 60 min (n = 14, group II) of left circumflex coronary artery occlusion. Seven animals of each group were studied 2 h and the others 4 weeks after flow was reinstated. After 2 h of reperfusion, regional myocardial function was absent in both groups. At 4 weeks regional function had returned to normal in group I, but was still significantly depressed in group II. Biochemical studies after 2 h of reperfusion showed that a functional index of the cardiac membrane, the in vitro cyclic AMP dependent 32P incorporation into phospholamban, was 71 (SEM 9)% compared to non-ischaemic myocardium in group I and 31 (6)% in group II (p less than 0.05). After 4 weeks this index had completely recovered in group I, 114 (13)%, but a significant decrease to 79 (2)% could still be observed in group II (p less than 0.05). After 2 h of reperfusion as well as after 4 weeks of recovery the myocytes in group II were more severely damaged than in group I. This study suggests that determination of in vitro phosphorylation of phospholamban shortly after reperfusion of ischaemic myocardium may be of value in the prediction of long term recovery of regional myocardial function.