SUMMARY.Right ventricular papillary muscles from normal rabbits and rabbits with sustained pulmonary artery constriction (67% decrease in external diameter) were studied at several resting muscle lengths and at an early instant in the isometric twitch. Instantaneous force-velocity data were obtained at 30-38% of time to peak tension (TPT) and at 96%, 98%, and 100% of the resting muscle length at which active twitch tension was maximal. Unloaded shortening velocity (Vmax) was estimated with a linearized form of the Hill hyperbolic formula, and was depressed in hypertrophy to 36% less than normal. We found that Vmaxdid not change with muscle length in the normal or hypertrophied muscles; therefore there was a length- and time-independent depression of contractile element shortening capacity that was consistent with previous work from this laboratory which demonstrated a depression of myosin and actomyosin ATPase activity in hypertrophy.