The protective effects of m-Nif 15, 30 and 50 micrograms.kg-1 and of Nif 30 micrograms.kg-1 on experimental myocardial infarction and metabolism in rabbits were evaluated. Blood samples were collected simultaneously from the femoral artery and the coronary sinus for examining lactate and potassium balance. In the normal group (n = 8) and the sham ligated group (n = 5), the lactate balance were +17 +/- 1.0 and + 13 +/- 1.1% respectively and the potassium balance were +2.1 +/- 0.08 and +2.4 +/- 0.08%, respectively. After occlusion of the LAD, the lactate and K+ balance reduced to -196 +/- 60.2 and -83.0 +/- 8.2% respectively, and m-Nif produced a dose dependent improvement of both balance. After giving m-Nif 15, 30 and 50 micrograms.kg-1 i.v., the lactate balance improved to -60 +/- 9, -48 +/- 6.7 and -24 +/- 1.8% and the K+ balance went up to -32.2 +/- 2.1, -17.6 +/- 0.1, -3.3 +/- 0.06, respectively. In the mean time, Nif 30 micrograms.kg-1 i.v. improved the lactate and K+ balances to -52 +/- 8.6% and -36.4 +/- 3.1%. In another experiment, propranolol 1 mg.kg-1 i.v. improved the lactate balance to -18 +/- 1.2% while the K+ balance increased to +2.0 +/- 0.05%. The area of the myocardial infarct was measured by nitroblue tetrazolium (N-BT) technique 24 h after LAD occlusion.2+ angina pectoris.