Intramyocardial pressure (IMP) and ventricular pressure (VP) were measured in the trabeculating heart of the stage-18 chick embryo (three days of incubation). Pressure was measured at several locations across the ventricle using a fluid-filled servo-null system. Maximum systolic and minimum diastolic IMP tended to be greater in the dorsal wall than in the ventral wall, but transmural distributions of peak active (maximum minus minimum) IMP were similar in both walls. Peak active IMP near midwall was similar to peak active VP, but peak active IMP in the subepicardial and subendocardial layers was 4 to 5 times larger. These results suggest that the passive stiffness of the dorsal wall is greater than that of the ventral wall and that, during contraction, the inner and outer layers of both walls generate more contractile force and/or become less permeable to flow than the middle part of the wall. The measured pressures likely correspond to regional variations in wall stress that may influence morphogenesis and function in the embryonic heart.