SUMMARY1. In attempting to consolidate the role of ventricular isomyosins in regulating the contractility of the myocardium, actomyosin ATPase and crossbridge kinetics were obtained at 24°C in chemically skinned isometrically contracting cardiac muscles containing V1and V3isomyosins.2. The ATPase activity was measured at various levels of Ca2+activation by the enzymatic coupling of ATP hydrolysis with the conversion of NADH to NAD+. The crossbridge kinetics were inferred from small‐amplitude perturbations of muscle length and muscle tension, and characterized by the frequency‐domain parameter fmin.3. The ATPase rates of V1and V3muscles obtained at various levels of Ca2+activation were plotted against the corresponding proportional tensions. The ATPasevstension plots were linear with slopes of 4.92 nmol/min‐1per mm per mN and 1.98 nmol/min‐1per mm per mN, respectively for, V1and V3muscles. Individual calculations of ATPase‐to‐tension ratios (nmol/min‐1per mm per mN) gave corresponding averages of 4.98 ± 0.12 (s.e.m.,n =12) and 2.16 ± 0.12 (s.e.m.,n =10). The myosin isoform induced proportional change in tension cost was accompanied by a similar change in fmin(4.1 ± 0.1 Hz and 1.95 ± 0.03 Hz, means ± s.e.m., for V1and V3muscles, respectively).4. The observations and other published kinetic data are discussed in the context of models of crossbridge cycling. It is suggested that the tension economy of V3muscle arises principally from an increase in the fraction of time, during the crossbridge cycle, when the crossbridg