To determine the ventricular looping pattern in relation to cardiac laterality, we studied rat embryos treated with retinoic acid (RA). A total of 243 Wistar rat embryos from an in vivo treated group (a single dose of 20-40 mg/kg all-trans RA administered to pregnant rats on day 6.5 to 9.5) and 29 control embryos were examined on day 13 of gestation. Twenty-nine embryos from the in-vitro treated group (treated by all-trans RA at 2 x 10(^-7) M for 6 hr on day 9.0 or 9.5 during the entire embryo culture for 72 hr) and seven control embryos were examined on day 12 of gestation. Abnormalities in cardiac laterality and ventricular looping were found in the in-vivo groups treated on day 8.5 and 8.75 and in the in-vitro group on day 9.0. Among 25 animals with abnormal laterality, right isomerism was the most common feature (22 cases), while the type of ventricular looping varied. Cases with normal laterality had a low incidence of abnormal looping (1.4%). In rat embryos treated with all-trans RA, normal cardiac looping was expected when cardiac laterality was normal. But in cases with abnormal laterality, the type of abnormal ventricular looping was unexpected.