Clinically equivalent doses of hydrocortisone, prednisolone, and dexamethasone have progressively increasing teratogenic activity as judged by their ability to induce cleft palate in the offspring of pregnant mice treated with these drugs during the middle period of gestation. Mole for mole, dexamethasone is at least 300 times more teratogenic than hydrocortisone. The enhanced teratogenicity of dexamethasone probably does not result from its relatively decreased mineralocorticoid activity.