We investigated the basic characteristics of the rat embryonic circulation and also looked at the hemodynamic effects of α- and β-agonists, digitalis, and atrial natriuretic peptide, using a modified organ culture system in which the embryo was placed in oxygenated Hanks' balanced salt solution, blood pressure was measured by a servo-null micropressure system, and blood flow pattern was obtained by a 20-MHz pulsed Doppler velocity meter. The peak pressure was 0.5 ± 0.04 (SEM) mm Hg at the atrium (n= 6), 2.3 ± 0.10 mm Hg at the ventricle (n = 15), 1.6 ± 0.03 mm Hg at the truncus (n= 7), and 1.0 ± 0.05 mm Hg at the umbilical artery (n= 21). There was a pressure drop from the ventricle to the truncus and then a smaller pressure decrease to the umbilical artery. The atrial a-wave was 20% of ventricular pressure and ventricular inflow blood flow pattern showed very low early-to-late filling ratio, indicating that the ventricle was stiff. These findings were essentially the same as in the chick embryo. We recorded the ventricular image by using a highspeed video system with a frame rate of 200/s, and the ventricular pressure-area loop showed a triangular shape with short isovolumetric phases, which was different from that of the chick embryo at a similar stage. Isoproterenol increased ventricular pressure from 2.3 ± 0.1 to 2.6 ± 0.1 mm Hg (n= 7,p< 0.05) and decreased umbilical blood pressure from 0.68 ± 0.05 to 0.61 ± 0.04 mm Hgn= 7,p< 0.05), suggesting contraction of the outflow tract, although positive change in pressure per unit time of ventricular pressure did not change. Ventricular negative change in pressure per unit time was increased with isoproterenol from 40 ± 4 to 51 ± 6 mm Hg/s (n= 7,p< 0.05), implicating improvement of diastolic function. Atrial natriuretic peptide and norepinephrine did not exert any significant responses, which was in contrast to the marked effects of these drugs seen in chick embryos. Acetylstrophanthidin, a rapid-acting digitalis, slowed the heart rate from 184 ± 4 to 171 ± 3 bpm (n= 6,p< 0.05) but did not change other parameters. In conclusion, basic characteristics of the circulation of the mammalian embryo are similar to those of the avian embryo, but responses to various cardiovascular agents known in the chick embryo cannot be extrapolated to the mammalian embryo.