A series of isotonic tension, shortening and shortening velocity were obtained in isolated rat papillary muscles by varying the ++set point force against which the muscle shortened at optimal length. It was found that (1) end systolic tension-length relation (ESTLR) could be described by an exponential equation. T = ae-bL-k, where a and k represent respectively the total tension and the resting tension and b means curved degree and length-axis intercept (L0) represents maximal shortening. There was a greater correlation index R2 when fitted with the equation compared with linear regression (P less than 0.001). (2) High [Ca2+]0 shifted ESTLR leftward and upward, with increased a, b and maximal shortening L0, so did norepinephrine (NE) but significantly less than high [Ca2+]0, whereas the changes in rates of shortening and tension rise were greater during NE infusion than during high [Ca2+]0 infusion. These results indicated that ESTLR was non-linear in isolated myocardium and exponential parameters a, b and L0 were sensitive to changes in contractile intensity, but less sensitive to changes in contractile velocity when compared with indexes of myocardial mechanics.