The causality of mutant sarcomeric proteins in hypertrophic cardiomyopathy (HCM) is well established. The current emphasis is to elucidate the pathogenesis of HCM in transgenic animal models. We determined the left ventricular ejection fraction (LVEF) in transgenic mice expressing mutant cardiac troponin T (cTnT)-Q⁹², known to cause HCM in humans. Transgenes were constructed by placing wild-type (R⁹²) or mutant (Q⁹²) full-length human cTnT cDNAs 3′ into a 5.5-kb murine [α -myosin heavy chain (MyHC)] promoter injected into fertilized zygotes. Three wild-type and six mutant lines were produced. Transgene mRNA and proteins, detected using transgene-specific probes were expressed at high levels in all wild-type and three mutant lines. The total cTnT mRNA pool was increased by up to five-fold in transgenic mice, but the total cTnT protein remained unchanged. The mean values of LVEF, determined by¹⁷⁸Ta radionuclide angiography, were 57.8±6% (n=4) in non-transgenic littermate (NLM), 53.3±10 (n=6) in wild-type and 39.4±6 (n=5) in mutant transgenic mice (P=0.009). The heart/body weight ratios and the number of cells stained with terminal deoxynucleotidyl transferase (TdT)-mediated nick end-labeling were similar among the groups. Three mutant mice had myocyte disarray and excess interstitial collagen and two had normal myocardial structure despite having reduced LVEF. Thus, in vivo expression of the mutant cTnT-Q⁹²protein, responsible for human HCM, impaired global cardiac systolic function in transgenic mice, which also occurred in the absence of myocyte disarray and increased interstitial collagen.