Medical management of patients with chronic left ventricular dysfunction continues to be a difficult problem. Recent clinical and experimental studies have suggested that 3,5,3'-triiodo-L-thyronine improves left ventricular pump function. However, whether 3,5,3'-triiodo-L-thyronine directly improves myocyte contractile function in cardiomyopathic states is unknown. Accordingly, this study examined the direct effects of 3,5,3'-triiodo-L-thyronine on isolated myocyte contractile function in cardiocytes obtained from control (n = 6) pigs and pigs with tachycardia-induced dilated cardiomyopathy (atrial pacing at 240 beats/min for 3 weeks; n = 6). Myocyte percent shortening and velocity of shortening were obtained at baseline and in the presence of 3,5,3'-triiodo-L-thyronine doses of 80 and 100 pmol/L. For both control and dilated cardiomyopathy groups, 3,5,3'-triiodo-L-thyronine caused a significant increase in myocyte contractile function. For example, a 100 pmol/L dose of 3,5,3'-triiodo-L-thyronine increased myocyte velocity of shortening by 51% in control myocytes and by 54% in dilated cardiomyopathy myocytes compared with baseline. A second series of experiments was performed to determine whether 3,5,3'-triiodo-L-thyronine altered the responsiveness of the beta-adrenergic receptor system in control and dilated cardiomyopathy myocytes. Myocyte contractile function was examined during beta-adrenergic stimulation with isoproterenol alone and in myocytes preincubated with 3,5,3'-triiodo-L-thyronine doses of 80 and 100 pmol/L to which isoproterenol was added. Isoproterenol alone increased velocity of shortening by 139% in control and by 233% in dilated cardiomyopathy myocytes compared with baseline. This was significantly greater than the increase with 3,5,3'-triiodo-L-thyronine alone. 3,5,3'-triiodo-L-thyronine followed by isoproterenol increased velocity of shortening by 245% in control and 313% in dilated cardiomyopathy myocytes compared with baseline. This was significantly greater than the response with 3,5,3'-triiodo-L-thyronine or isoproterenol alone and appeared to be greater than an additive response. The results from this study clearly demonstrated that 3,5,3'-triiodo-L-thyronine directly augmented myocyte contractile function in both control and dilated cardiomyopathy myocytes. In addition, 3,5,3'-triiodo-L-thyronine enhanced the contractile response to beta-adrenergic stimulation in dilated cardiomyopathy. This study provides unique evidence to suggest that 3,5,3'-triiodo-L-thyronine may be a useful adjunct to conventional inotropic support in the setting of advanced left ventricular dysfunction.