Heart rate (HR), developed pressure, and stroke volume (SV) increase dramatically during cardiac morphogenesis. During acute changes in cycle length (CL), the chick embryo maintains cardiac output by a compensatory change in SV. We determined embryonic pressure-volume relations during alterations in CL in the stage 21 while Leghorn chick embryo to quantitate the relationship between CL and cardiac time intervals, ventricular preload, and ventricular function. CL was transiently altered to 73-191% of initial baseline CL with a 1-mm randomly preheated or cooled metal probe applied to the sinus venosus. Isometric relaxation, diastolic filling, and systolic ejection times varied linearly with CL, whereas isometric contraction times were similar. SV varied linearly with CL, ejection time, and end-diastolic volume. In contrast to the mature heart, the relationship between SV and end-systolic pressure was inverse. Embryonic ventricular preload was influenced acutely by diastolic filling time, and cardiac output was optimized during changes in CL by ventricular-vascular interactions that matched SV inversely to HR.