Objective: The purpose of this study was to determine the effects of pregnancy and of maximal exercise on left ventricular diastolic filling response. Study Design: Transmitral pulsed Doppler echocardiography was obtained in 10 healthy women during each trimester of pregnancy and at 12 weeks after delivery. Doppler studies were performed at rest and at each exercise workload. The P-R interval, the early and atrial peak flow velocities, the mitral early deceleration time, and the isovolumetric relaxation time were analyzed. Data are expressed as the mean and standard deviation of the mean. Values obtained during the last trimester of pregnancy were used as the pregnant value; values at the 12 weeks after delivery were used as the nonpregnant value. Paired t-test, analysis of variance, and mixed models were used to determine significance with a probability value of <.05. Results: Pregnancy significantly increased the early and atrial peak flow velocities. Pregnancy decreased the P-R interval, the early deceleration time, and the isovolumetric relaxation time. Exercise significantly decreased these diastolic functions; but pregnancy, in any of the 3 trimesters, did not significantly affect this response. Conclusion: Pregnancy increased left ventricular diastolic camber stiffness at rest and shifted left ventricular diastolic filling during exercise from predominantly early to atrial filling. This finding suggests that there is an increase in left ventricular chamber stiffness during maximal upright bicycle exercise in pregnancy. (Am J Obstet Gynecol 2001;185:822-7.)