We studied the interrelation of changes in ventricular external work, myocardial blood flow, and O2 consumption at birth. Eleven fetal sheep were instrumented under general anesthesia at 133-134 days gestation with arterial, peripheral venous, and left atrial catheters, and in five fetuses, a catheter was also inserted into the coronary sinus. Fetal left ventricular (LV) and right ventricular (RV) outputs and myocardial blood flows (radioactive microspheres), hemodynamics, and LV O2 consumption were measured 1 wk later. Fetuses were delivered by cesarean section, and measurements were repeated 1 and 4 h after birth. RV minute work (mmHg.l.kg-1) was greater in fetuses (10.8 +/- 0.5 vs. 15.6 +/- 1.0 P < 0.001), but because of increased LV minute work (to 20.9 +/- 1.4, P < 0.005) and unchanged RV minute work, this pattern was reversed by 1 h after birth. RV myocardial blood flow (ml.min-1.100 g-1) was predominant in fetuses (234 +/- 25 vs. 306 +/- 27, P < 0.001), but this also reversed in lambs; this reversal was related to unchanged 1-h (248 +/- 16) and lower 4-h LV myocardial blood flow (199 +/- 9, P < 0.05) and a progressive fall in RV myocardial blood flow (1 h: 245 +/- 20, P < 0.05; 4 h: 174 +/- 12, P < 0.005). LV O2 consumption (ml.min-1.100 g-1) increased between fetal (8.1 +/- 0.6) and 1-h lambs (18.9 +/- 1.3, P < 0.005) because of a rise in the LV arteriovenous O2 content difference (3.6 +/- 0.4 vs. 7.5 +/- 0.6 ml/dl, P < 0.005). External work performed per unit myocardial blood flow (mmHg.l.ml-1) in the LV increased between fetal and 1-h lambs (1.92 +/- 0.14 to 3.33 +/- 0.15, P < 0.001) and, in the RV, between fetal and 4-h lambs (2.09 +/- 0.18 to 2.92 +/- 0.24, P < 0.01). The proportion of consumed O2 converted to external work was, however, similar in fetal (34.6 +/- 3.1%) and 1-h (31.3 +/- 2.9%) and 4-h lambs (34.5 +/- 3.6%). These findings indicate that 1) a switch from a fetal RV to a newborn LV dominance is due to increased LV pumping performance, 2) an associated switch in LV and RV myocardial blood flow patterns is mainly related to a fall in RV myocardial blood flow, 3) a postnatal rise in LV O2 consumption results from enhanced LV arteriovenous O2 extraction, not increased LV myocardial blood flow, 4) ventricular external work per unit myocardial blood flow increases postnatally, particularly in the LV, and 5) LV mechanical efficiency is not altered after birth.