Little is known about the hemodynamic properties of the rapidly expanding arterial bed during embryonic development. Using a servo-null pressure system and 20-MH<sc>z</sc> pulsed Doppler velocity meter, we recorded simultaneous dorsal aortic pressure and velocity waveforms. The waveforms were digitized at 3-msec intervals and subjected to Fourier analysis. We calculated hydraulic energy and the impedance spectrum to 10 H<sc>z</sc>. From stages 18 to 29, heart rate (148±3 to 193±9 beats/min), systolic pressure (1.14±0.12 to 3.04±0.10 mm Hg), and mean dorsal aortic blood flow (21 ±2 to 214±19 mm3/min) increased. Peripheral vascular resistance (Z0: 30.4±4.8 to 6.4±0.7 dyneXsec/mm5), and the impedance moduli (Z1: 6.5±1.0 to 1.7±0.2 dyneYsec/mm5; Z2:6.1±1.2 to 1.7±0.1 dyneXsec/mm5; Z3: 7.3±1.1 to 1.7±0.2 dyneXsec/mm5) decreased. Total hydraulic power increased from 48±7 to 2,606±96 nW, while the proportion of oscillatory energy increased from 29±2percent; to 65±4percent;. With development hydraulic load decreases, total external work increases and the dorsal aorta and embryonic vascular bed becomes more compliant. A greater proportion of total energy is expanded in pulsatile blood flow, suggesting that ventricular-arterial coupling is less efficient later in development.