Aims: Muscle LIM protein (MLP) null mice are often used as a model for human dilated cardiomyopathy. So far, little is known about the time course and pathomechanisms leading to the development of the adult phenotype. Methods and results: We systematically analysed the contractile phenotype, myofilament calcium (Ca ²) responsiveness, passive myocardial mechanics, histology, and mRNA expression in mice aged 4 and 12 weeks. In 4-week-old animals, there was no significant difference in the forcefrequency relationship (FFR) and catecholamine response of intact isolated papillary muscles between wild-type (WT) and MLP null myocardium. In 12-week-old animals, WT myocardium exhibited a significantly positive FFR, while that of MLP null mice was significantly negative, and the inotropic response to catecholamines was significantly reduced in MLP null mice. This time course of decline in contractile function was confirmed in vivo by echocardiography. Whereas at 4 weeks of age MLP null mice and WT littermates showed similar levels of SERCA2a (sarcoplasmic reticulum Ca ² ATPase) expression, the expression was significantly lower in 12-week-old MLP null mice compared with littermate controls. Myofilament Ca ² responsiveness was not affected by the lack of MLP, irrespective of age. Whereas in 4-week-old animals MLP null myocardium showed a trend to an increased compliance compared with the WT, myocardium of 12-week-old MLP null mice was significantly less compliant than WT myocardium. Parallel to the decrease in compliance there was an increase in fibrosis in the MLP null animals. Conclusion: Our data suggest that MLP deficiency does not primarily influence myocardial contractility. A lack of MLP leads to an age-dependent impairment of excitationcontraction coupling with resulting contractile dysfunction and secondary fibrosis. Published on behalf of the European Society of Cardiology. All rights reserved.