AbstractThe mean ventricular blood pressure (MVBP) of 141 control rat embryos and 164 embryos exposed to trypan blue was measured at 13.5, 14.5, 17.5, and 18.5 days of gestation. The mean pressure in the controls ranged from 19.7 mm water at 13.5 days to 62.7 mm at 18.5 days. The mean pressure of the treated embryos was the same as that of the controls at 13.5 days but about 10 mm lower at all other ages. The range of variation was much greater in experimental embryos than in controls. Many extremely low pressures were recorded in the treated embryos. The pressure differences between control and treated embryos were not due to any differences in rate of heart beat. Since the morphogenesis of the heart and major blood vessels is not completed until day 15.5, and because abnormal pressure and flow patterns can affect cardiovascular development, the abnormal pressures in the 13.5‐ and 14.5‐day groups possibly contribute to the high rate of cardiovascular anomalies induced by trypan blue. The abnormal pressures in the older age group are probably reflections of this heart dam