Objective A number of cardiovascular variables exhibit a circ adian rhythm. Whethe r myocardial contractile response and gene expression of the contractile protein also show changes with a similar period was here investigated. Method Circadi an variabilities in the left ventricular developed pressure (LVP) and contractil ity (LV dp/dt max) were measured in 24 Sprague-Dawley r ats by directly left ve ntricular catheterizing and compared with changes in the gene expression of α- myosin heavy chain (α-MHC) in myocytes obtained from the same animals by dot b lottin g analysis. Results A circadian rhythm was seen in the variabili ty of LVP (P<0.001), LV dp/dt max (P<0.001) and the bio chemically measured expression of the α- MHC gene (P<0.01). As compared to the amplitude of the rhythm i n α-MHC gene exp ression, the amplitude of the contractility rhythm was large (P< 0.01) and the ci rcadian amplitude of the LVP(P<0.001) was the largest, represent ing perhaps a co mposite of intracardiac plus any extracardiac contributions. Conclusion One of factors determing the circadian rhythm of myocardial contractile function is α -MHC gene expression level.