Global myocardial ischemia and reperfusion injury play a major role in early postoperative graft dysfunction. In the present study, the influence of nitric oxide (NO) on ischemia/reperfusion injury and catecholamine sensitivity was investigated in a heterotopic rat heart transplantation model. After one hour ischemic preservation, reperfusion was started either after application of placebo (n = 8, control) or nitro-L-arginine methyl ester (L-NAME, 10 mg/kg, n = 8) for inhibition of NO synthesis. Myocardial blood flow (MBF), left ventricular pressure (LVP), its first derivative (dP/dt), end diastolic pressure (LVEDP) and isovolumic relaxation constant (T e ) were measured after 60 minutes reperfusion and with subsequent dobutamine (D) infusion (5 μg/kg/min). One-day graft survival was decreased after L-NAME (5/8 after L-NAME vs 8/8 in control). MBF was significantly decreased after L-NAME (1.2 +/- 0.1 vs 1.9 +/- 0.2 ml/min/g, p < 0.05). The L-NAME group showed a decreased systolic function (maximal LVP: 53.2 +/- 6 vs 86.5 +/- 7 mmHg, p < 0.05; maximal dP/dt: 1257 +/- 290 vs 3410 +/- 257 mmHg/s, p < 0.05) and a significant leftward shift of the LVEDP-left ventricular volume curve (LVEDP: 12.57 +/- 1.4 vs 4.76 +/- 0.6 mmHg, p < 0.05 at a reference volume of 80 μl). Myocardial relaxation was prolonged after L-NAME (T e : 23.8 +/- 3.1 vs 11.3 +/- 1.6 ms, p < 0.05). D increased LVP, dP/dt and T e in both groups, however, this increase was smaller in the L-NAME group (p < 0.05). MBF remained unchanged with D in the L-NAME group and increased in the control group (1.2 +/- 0.2 vs 2.7 +/- 1.9 vs ml/min/g, p < 0.05). These results indicate that 1) NO production within the graft during reperfusion represents a substantial determinant of graft function, 2) NO formation plays an important role in β-adrenergic responses of the microvasculature in the graft after heart transplantation.