Rat embryos explanted at stages between primitive streak and head-fold were exposed in rotating bottle cultures to glucose levels raised by 3-15 mg/ml either throughout a 66-h culture period, or for shorter intervals near the start of culture (with the rest of the culture period at normal glucose levels). A high proportion of embryos explanted at 8 days 20 h or at 9 days and exposed to 12 mg/ml exogenous glucose for only the first or second 8-h periods of a 66-68-h culture were malformed, whereas embryos exposed for the third 8 h were relatively resistant, and embryos exposed later still were unaffected. There were indications that embryos at stages of development corresponding to the second 8-h periods in these cultures were particularly susceptible to hyperglycaemia. Embryos explanted at 9 days exhibited malformations when exposed to 6-9 mg/ml glucose throughout culture, compared with the 12-15 mg/ml shown previously to be necessary to induce anomalies in embryos explanted 12 h later.