The study demonstrates relationships between changes in the fluid environment around the embryo and appearance of anomalies induced by Trypan blue. Rat embryos were exposed in utero or in vitro during the early period of organogenesis. Both exposures resulted in defects which are typical of trypan blue treatment. Osmolality of exocoelomic fluid (ECF) was measured on gestation day 10 in vivo and day 12 in vitro. In both cases ECF osmolality was significantly lower than controls. This was correlated with the presence of edema-related anomalies in the embryo. On gestation day 11 in vivo, three days after maternal injection of trypan blue, ECF osmolalities were significantly higher than controls; however, there was tremendous variability in this parameter in day 11 treated embryos, and some had ECF osmolalities below the control range. Increased frequency of abnormalities was correlated with abnormal ECF osmolality, below and above the control range. The movements of an amino acid and a monosaccharide across the visceral yolk sac were measured on gestation day 12 embryos in vitro.