There have been no investigations of chronologic changes in postnatal contractile state of both left and right ventricles of the newborn. Thus, we determined isovolumetric indices of contractility in 32 acutely instrumented newborn lambs (1 day to 12 wk; three exteriorized near term fetuses, and four adult sheep. These studies demonstrated a marked increase in inotropic properties of both ventricles which was especially pronounced during the first 3 days of extrauterine life (group 1, left ventricle Vmax 3.2 +/- 0.27 and VCE10 3.2 +/- 0.28 Ml (muscle lengths)/sec; right ventricle Vmax 2.1 +/- 0.2 and VCE10 2.2 +/- 0.13 ML/sec); and was associated with elevated cardiac output (428 +/- 52 cc/kg/min), largely resulting from an elevated stroke volume (2.1 +/- 0.31 cc/kg). During subsequent postnatal maturation, contractility indices of both ventricles gradually declined to values intermediate between newborn and adult ((group IV, 22 to 84 days) - left ventricle Vmax 1.78 +/- 0.13 and VCE10 1.79 +/- 0.15 ML/sec; right ventricle Vmax 1.48 +/- 0.07 and VCE10 1.31 +/- 0.3 ML/sec); associated with a corresponding decline in cardiac output (group IV, 136 +/- 38 cc/kg/min), related in large measure to a decline in stroke volume (0.76 +/- 0.13) cc/kg). Simulation of birth in exteriorized fetuses was associated with a sudden increase in contractile indices for both left (Vmax 2.1 +/- .2 leads to 3.0 +/- .3 and VCE10 2.1 +/- 0.2 leads to 3.1 +/- 0.3 ML/sec) and right (Vmax 2.0 +/- 0.2 leads to 2.5 +/- 0.2 and VCE10 1.9 +/- 0.1 leads to 2.5 +/- ML/sec) ventricles. We conclude that there is a postnatal elevation of ventricular contractile state which contributes to successful postnatal adaptation of the newborn cardiovascular system.