Passive stiffness and hydroxyproline content of myocardium hypertrophied by pressure-loading were determined in kittens 2, 8-16, and 24-52 wk after pulmonary artery banding, which initially elevated right ventricular systolic pressure by 10-15 mm Hg. Right ventricular mass increased by approximately 75%, three-quarters of which occurred during the first 2 wk after banding. Passive stiffness was assessed from resting length-tension relations of isometrically contracting isolated right ventricular papillary muscles. Stiffness constants, alpha and beta were determined from the relationship sigma = alpha (e beta epsilon - 1) where sigma = stress and epsilon = Lagrangian strain. Elastic stiffness (d sigma/d epsilon) was derived from: d sigma/d epsilon = beta sigma + beta alpha. Right ventricular hydroxyproline increased in proportion to muscle mass so that hydroxyproline concentration remained unchanged after banding. Both alpha, beta, and elastic stiffness-stress relations were similar to values in nonbanded controls. Thus, we did not observe an increase in passive stiffness or hydroxyproline concentration of pressure-stiffness or hydroxyproline concentration of pressure-induced hypertrophied myocardium in contrast to most previous studies.