A 3-month-old child with bilateral cleft lip and palate and holoprosencephaly was hospitalized after he developed diabetes insipidus presumably due to hypothalamic dysfunction. He was initially treated with subcutaneous vasopressin injection but was switched to therapy with desmopressin acetate (DDAVP) before discharge. Because of his abnormal nasopharyngeal anatomy, we decided to administer the desmopressin acetate sublingually, and this was effective. A single daily dose of 2 micrograms (0.4 microgram/kg) resulted in a prompt antidiuresis, and the effect gradually lessened over a 24-hour period. Serum electrolyte values were restored to normal and have remained normal after three months of treatment. After additional study, the sublingual route might be considered for the administration of small-polypeptide therapeutic agents when other routes are impractical.