Although atrial morphologic changes are well documented, the description of early atrial function is limited. We used videomicroscopic methods to define the function of the contracting atrium in stage 16 to 24 white Leghorn chick embryos. We exposed the embryo in ovo (right side up) and imaged the ventricle, then repositioned the embryo (left side up) and imaged the atrium (n≥ 8 per stage). We traced the atrial endocardial border and then measured atrial perimeter (mm) and cross-sectional area (mm2). A 20-MHz pulsed Doppler velocity meter was used to measure atrioventricular blood velocity during atrial imaging in an additional six stage 21 embryos. Data were tested by analysis of variance and regression analysis. Mean heart rate change after repositioning was −4 ± 1%. Atrial maximum and minimum area increased linearly versus embryo stage (y= 0.10x− 1.41,r= 0.89,p< 0.05 and y = 0.05x− 0.67,r= 0.82,p< 0.05, respectively). Shortening fraction (percentage of reduction) of atrial perimeter and area decreased from 32.3 ± 2.0% to 27.5 ± 1.8% (p< 0.05) and 56.2 ± 3.0% to 47.7 ± 2.0% (p< 0.05), respectively, from stage 16 to 24. During atrial contraction, the velocity of circumferential wall shortening increased linearly with stage (y= 0.22x− 2.08,r= 0.81,p< 0.01); however, the velocity of lengthening was similar between stages (p= 0.45). Simultaneous atrial imaging and pulsed Doppler velocity measurement showed that passive atrioventricular flow occurred late in atrial lengthening and active atrioventricular flow occurred during atrial contraction. Thus, atrial function increases in parallel with morphogenesis during early cardiac development, and measures of atrial function can now be incorporated into a physiologic model of the developing cardiovascular system. (Pediatr Res 32: 333–337, 1992)