Fifteen week old male Wistar rats (n=7) were made diabetic by intravenous injection of streptozotocin (50mg/kg). Agematched, untreated male Wistar rats (n=9) served as controls. Hearts were removed after 5-6 weeks of diabetes, and the isometric developed tension (T) of isolated left ventricular papillary muscles and its first derivative (dT/dt) were measured at a frequency of 0.2Hz. During testing, the muscles were perfused with Tyrode's solution (Ca2⁺ concentration was half of normal Tyrode's solution, pH7.4, 32°, bubbled with 95% O² and 5% CO₂). In addition, the left ventricular isoenzyme pattern, which is related to myocardial energetics, was determined by pyrophosphate gel electrophoresis. There was no significant difference in isometric developed tension between diabetic and control rats (DM: 2.90±0.89 vs controls: 2.87±0.85g/mm², mean±SD), but in diabetic rats, dT/dtmax decreased significantly as compared with controls (DM: 23.5±4.2 vs controls: 31.9±7.9g/mm²•s, p<0.05). Myocardial mechanical responses to isoproterenol (10-⁷M) and dibutyryl cyclic AMP (10-⁵M) also decreased in diabetic rats. The left ventricular myosin isoenzyme pattern shifted toward VM-3 in diabetic rats (VM-3: DM: 74.9±10.7 vs controls: 9.5±4.1%, p<0.001). These results indicate that diabetes influences myocardial contractility and changes cardiac energetics. Postreceptor processes may play a role in myocardial mechanical responses to catecholamines in streptozotocindiabetic rats.