Rabbits, ages 4-24 weeks, were injected with saline or thyroxine (150 micrograms/kg) for 7 days, and force-velocity curves were generated using papillary muscles from these hearts by a method described previously. In addition, the structure and relative amounts of myosin isozymes from papillary muscles and from 3- to 5-mg segments of the left and right ventricular free wall were analyzed by polyacrylamide gel electrophoresis under native and denatured conditions. We found that rabbit papillary muscles may contain up to three isozymic forms of myosin (V1, V2, and V3) and that their relative amounts change with age of the rabbit and with thyroxine treatment. There were no differences between papillary muscles and ventricular free wall in the molecular weight of light chain1 (27,000) and light chain2 (21,500) of heavy chain alpha or in the peptide map of heavy chain alpha nor were there any differences between papillary muscles and the ventricular free wall in the molecular weight of light chain1 (27,000) and light chain2 (21,500) of heavy chain beta or in the peptide map of heavy chain beta. However, the relative amounts of myosin isozymes in the ventricular free walls and papillary muscles may not be identical within the same heart. Analysis of the force-velocity curves indicated that the speed of papillary muscle shortening is correlated with the relative amount of V1 myosin present in each papillary muscle. Papillary muscles that contain 100% V1 myosin shorten, under zero load, approximately six times faster than papillary muscles that contain 100% V3 myosin. Our results indicate that changes in the relative amounts of myosin isozymes are responsible, at least in part, for sustained alterations in the speed of papillary muscle shortening.