Rat embryos have been cultured in vitro from 9.5 days of gestation for different times in serum containing mannose, and the embryos have been observed by scanning electron microscopy. Embryos cultured in 3 mg/ml (1.7 X 10(-2) M) or 6 mg/ml (3.3 X 10(-2) M) mannose for 48 h showed inhibition of the expansion of the yolk sac and were smaller than the control embryos. Mannose-treated embryos also showed delayed development according to morphological criteria, and a range of abnormalities including abnormalities of the neural tube. Embryos cultured in 6 mg/ml mannose for 24 h also showed significant inhibition of yolk-sac expansion and were smaller and less advanced than the control embryos. Abnormalities were seen, including a delay in the closure of the neural folds. Abnormalities were also observed in embryos cultured in mannose for 10 h; embryos at the neural groove stage showed irregularities in the neural groove. Mannose did not inhibit the re-elevation of neural folds which had been caused to collapse by exposure to medium containing low calcium. These results are compatible with the idea that mannose retards development and thereby perturbs the morphogenesis of the neural tube.