In this study we investigated whether the expression of N-terminal myosin light chain-1 (MLC-1) peptides could improve the intrinsic contractility of the whole heart. We generated transgenic rats (TGR) that overexpressed minigenes encoding the N-terminal 15 amino acids of human atrial MLC-1 (TGR/hALC-1/1-15, lines 7475 and 3966) or human ventricular MLC-1 (TGR/hVLC-1/1-15, lines 6113 and 6114) isoforms in cardiomyocytes. Synthetic N-terminal peptides revealed specific actin binding, with a significantly (P<0.01) lower dissociation constant (K(D)) for the hVLC-1/1-15-actin complex compared with the K(D) value of the hALC-1/1-15-actin complex. Using synthetic hVLC-1/1-15 as a TAT fusion peptide labeled with the fluorochrome TAMRA, we observed specific accumulation of the N-terminal MLC-1 peptide at the sarcomere predominantly within the actin-containing I-band, but also within the actin-myosin overlap zone (A-band) in intact adult cardiomyocytes. For the first time we show that the expression of N-terminal human MLC-1 peptides in TGR (range: 3-6 muM) correlated positively with significant (P<0.001) improvements of the intrinsic contractile state of the isolated perfused heart (Langendorff mode): systolic force generation, as well as the rates of both force generation and relaxation, rose in TGR lines that expressed the transgenic human MLC-1 peptide, but not in a TGR line with undetectable transgene expression levels. The positive inotropic effect of MLC-1 peptides occurred in the absence of a hypertrophic response. Thus, expression of N-terminal domains of MLC-1 represent a valuable tool for the treatment of the failing heart.