The linear time-varying elastance theory is frequently used to describe the change in ventricular stiffness during the cardiac cycle. The concept assumes that all isochrones (i.e. curves that connect pressure-volume data occurring at the same time) are linear and have a common volume intercept. Of specific interest is the steepest isochrone, the end-systolic pressure-volume relationship (ESPVR), of which the slope serves as an index for cardiac contractile function. Pressure-volume measurements achieved with a combined pressure-conductance catheter in the left ventricle of 13 open-chest anesthetized mice showed a marked curvilinearity of the isochrones. We therefore analyzed the shape of the isochrones using 6 regression algorithms (2 linear, 2 quadratic and 2 logarithmic, each with a fixed or time-varying intercept) and discussed the consequences for the elastance concept. Our main observations were: (i) the volume intercept varies considerably with time; (ii) isochrones are equally well described using quadratic or logarithmic regression; (iii) linear regression with a fixed intercept shows poor correlation (R²<0.75) during isovolumic relaxation and early filling, and (iv) logarithmic regression is superior in estimating the fixed volume intercept of the ESPVR. In conclusion, the linear time-varying elastance fails to provide a sufficiently robust model to account for changes in pressure and volume during the cardiac cycle in the mouse ventricle. A new framework accounting for the non-linear shape of the isochrones needs to be developed.