To characterize developmental changes in impulse propagation within atrial musculature, we performed high-speed optical mapping of activation sequence of the developing chick atria using voltage-sensitive dye. The activation maps were correlated with detailed morphological studies using scanning electron microscopy, histology, and whole mount confocal imaging with three-dimensional reconstruction. A preferential pathway appeared during development within the roof of the atria, transmitting the impulse rapidly from the right-sided sinoatrial node to the left atrium. The morphological substrate of this pathway, the bundle of Bachman, apparent from stage 29 onward, was a prominent ridge of pectinate muscles continuous with the terminal crest. Further acceleration of impulse propagation was noted along the ridges formed by the developing pectinate muscles, ramifying from the terminal crest toward the atrioventricular groove. In contrast, when the impulse reached the interatrial septum, slowing was often observed, suggesting that the septum acts as a barrier or sink for electrical current. We conclude that these inhomogeneities in atrial impulse propagation are consistent with existence of a specialized network of fast-conducting tissues. The purpose of these preferential pathways appears to be to assure synchronous atrial activation and contraction rather than rapid impulse conduction between the sinoatrial and atrioventricular nodes.