The distribution of the vagal nerve was studied in whole-mount specimens and serial sections of chick embryos after retinoic acid treatment. White Leghorn chick embryos were treated at stage 15 either with 1 microg all-trans retinoic acid (n=11), or with the solvent dimethylsulphoxide (sham-operated embryos, n=8). Eight embryos served as normal controls. At stage 34 all 27 embryos were examined with a dissecting microscope. In order to reveal the vagal patterning, the hearts were removed and whole-mount stained with the HNK-1 antibody. In three hearts of the retinoic acid-treated group a morphologic intracardiac anomaly -- a double outlet right ventricle -- was found. To explore in depth the vagal nerve distribution in the heart, a separate set of hearts of retinoic acid embryos (n=5), sham-operated (n=4) and control embryos (n=5), was devised solely for serial sectioning and staining with the HNK-1 antibody. All hearts of retinoic acid-treated embryos showed a disturbed vagal nerve distribution both over the surface of the heart and within the heart wall. The vagal patterning was not altered in the sham-operated embryos compared to controls. It is concluded that retinoic acid disturbs the development of vagal nerve patterning regardless of the concurrent presence of intracardiac malformations. The mechanism and functional implications remain to be investigated.