Diabetes mellitus (DM) has been implicated in the development of cardiomyopathy even in the absence of coronary disease. The reduced contractile performance is associated with disturbed Ca2+i handling in the manifest stage of the diabetes. Very little is known, however, about alterations of intracellular Ca2+i in the early phase of DM. This study was designed to elucidate the functional and tissue level changes of the major Ca2+i homeostasis proteins by analyzing the optically measured Ca2+i transient and determining the expression level of the sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), Na+-Ca2+ exchanger, phospholamban and the ryanodin channel (RyR2), respectively, in Langendorff perfused hearts isolated from 4 and 6 weeks Type I diabetic rats. We showed that during 4–6 weeks of DM no major changes occur in the expression levels of the examined transport and channel proteins. The analysis of the Ca2+i transient in the 4-week DM hearts revealed no apparent changes in Ca2+i handling in the resting state. Responses of SERCA2a and RyR2 to beta-adrenergic activation were diminished accompanied by lesser response of inotropy and lusitropy. Six weeks DM resulted in reduced inotropy, lusitropy accompanied by decreased maximal transport capacity of SERCA2a and conductance of RyR2 both during rest and beta-adrenergic activation. In conclusion it appears that between 4 and 6 weeks of DM there is a transition from adaptive to maladaptive phase of the disease.