Several inotropic interventions were studied in thin papillary muscles under dynamic conditions. The effects on mechanical restitution and postextrasystolic potentiation were analysed. The decay of postextrasystolic potentiation was taken as a measure of recirculation fraction of activator calcium. The mechanical restitution curve had a plateau phase on its rising phase which was abolished in low extracellular sodium but pronounced in increased extracellular calcium. The recirculation fraction (RF) in control was 0.35 +/- 0.03; lowering the extracellular sodium by 20% increased the RF to 0.46 +/- 0.04 (n = 10). A reduction of sodium by 40% increased the RF to 0.57 +/- 0.04, whereas increasing extracellular calcium to 4 mM gave an RF of 0.48 +/- 0.05 (n = 10 in all cases). There was no significant effect on RF of changing basic stimulation frequency or muscle preparation length. These findings support RF as a good index of myocardial contractility. Furthermore, at muscle diameters above 0.65 mm the RF was found to be reduced, suggesting this diameter as critical for muscle function. Also, postextrasystolic potentiation in relation to preceding steady state contraction was markedly increased at these diameters. In conclusion, this study shows that RF is independent of stimulation frequency and muscle length, and that it is increased when calcium extrusion by the sodium/calcium exchange is reduced. Furthermore, RF is critically dependent upon the diameter of the preparation and mechanical restitution is changed by altered extracellular sodium concentration.