The objective of this paper is to document that the fetal heart has the capacity to increase stroke volume in the face of complete heart block. Measurements of left and right ventricular fractional shortening (an index of ventricular function that correlates with stroke volume) were obtained from three fetuses with complete heart block and no structural defects. Measurements were obtained antepartum and postpartum using M-mode sonography and compared to published measurements from normal fetuses. All three fetuses showed an increase in fractional shortening over normal values. The increase was unequal between ventricles and was maintained postpartum. These three fetuses were able to increase their fractional shortening (stroke volume) in response to persistent bradycardia. This supports the theory that the Frank-Starling mechanism is operational in the fetal heart. © 1995 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.