This study was performed to determine the relationship between variations of arterial cross-section and cardiac period. Relative section variations were recorded by impedance rheography. In 176 anesthetized adult mongrel dogs, aortic and femoral rheograms, femoral pressure, and ECG were recorded. The cardiac period (Tc) and the interval between the systolic and the dicrotic wave, i.e., the arterial intrinsic period (Ta), were measured and related to each other (Tc/Ta ratio). In 125 dogs, the dicrotic wave appeared spontaneously. In these cases Tc/Ta 1.98 +/- 0.16. In 51 dogs, no dicrotic wave appeared spontaneously. Lengthening of cardiac period was obtained in 25 of these dogs by vagus stimulation. As related to the cardiac period before vagal stimulation Tc/Ta was then 0.98 +/- 0.08. In the remaining 26 dogs, such a lengthening was obtained by inducing a premature ventricular depolarization. In this group, Tc/Ta was 1.00 +/- 0.03. The results of the present study suggest that a constant adaptation exists between ventricular ejection and arterial dynamics. The physiologic implications are discussed.