Objective: Acute inotropic effects of triiodothyronine (T3) have been reported, employing both in vivo experimental animal models and in vitro isolated heart perfusions. However, the mechanisms responsible for these acute inotropic effects remain unclear. The aim of this study, therefore, was to delineate the role of the β-adrenergic receptor system in these acute responses. Methods: The hearts from both euthyroid and hypothyroid (treated with 0.05% PTU in drinking water) male Sprague-Dawley rats were used in 5 experimental study protocols. Hearts from euthyroid rats were perfused with buffer containing either T3 (10-7 M) or control while continuously recording left ventricular function for 10 min ('acute effects'). Two-hour perfusions ('subacute effects') and cardiac responses following increasing doses of isoproterenol (10-10 to 10-6 M) in the presence or absence of T3-containing buffer (acute interaction) were also determined. In hypothyroid rats, the subacute responses and the acute interactions were investigated. Results: In the presence of T3, an acute, significant potentiation of the inotropic responses following β-adrenergic stimulation with isoproterenol was observed in both rat cohorts, which was more pronounced in hearts from euthyroid rats. An acute (< 40 s), but transient (79 ± 8 s), direct inotropic response was observed in hearts from euthyroid rats. No cardiac responses were seen during a 2-h perfusion in hearts from either euthyroid or hypothyroid rats. Conclusions: The acute inotropic effects of T3 in non-ischemic myocardium probably result from an acute interaction between T3 and catecholamines rather than through a direct inotropic effect of T3 alone.