Cardiac hypertrophy in response to systolic pressure overloading frequently results in contractile dysfunction, the cause for which has been unknown. Since, in contrast, the same degree and duration of hypertrophy in response to systolic volume overloading does not result in contractile dysfunction, we postulated that the contractile dysfunction of pressure hypertrophied myocardium might result from a direct effect of stress as opposed to strain loading on an intracellular structure of the hypertrophied cardiocyte. The specific hypothesis tested here is that the microtubule component of the cytoskeleton is such an intracellular structure, which, forming in excess, impedes sarcomere motion. The feline right ventricle was either pressure overloaded by pulmonary artery banding or volume overloaded by atrial septotomy. The quantity of microtubules was estimated from immunoblots and immunofluorescent micrographs, and their mechanical effects were assessed by measuring sarcomere motion during microtubule depolymerization. We show here that stress loading increases the microtubule component of the cardiac muscle cell cytoskeleton; this apparently is responsible for the entirety of the cellular contractile dysfunction seen in our model of pressurehypertrophied myocardium. No such effects were seen in right ventricular cardiocytes from normal or volume-overloaded cats or in left ventricular cardiocytes from any group of cats. Importantly, the linked microtubule and contractile abnormalities are persistent and thus may be found to have significance for the deterioration of initially compensatory cardiac hypertrophy into the congestive heart failure state.