The investigator thinks that the oxytocin (OT) can improve durably and significantly the behavior disorders and thus the socialization but also the satisfaction and could thus be an interesting therapeutic alternative for the patients presenting a Prader-Willi Syndrome (SPW). Although today several studies demonstrated the effects of the OT in various domains of the behavior, the investigator do not know either its specificity of action about the cerebral level, or its duration of action, or the optimal modalities of administration and in particular at patients SPW.

Inclusion Criteria:

• Prader-Willi syndrome genetically confirmed
• Absence of extension of the QT interval in the electrocardiogram
• Absence of hypokalemia

Exclusion Criteria:

• Psychiatric troubles
• Anomalies of the heart rhythm in significant ECG
• Hepatic insufficiency
• Renal insufficiency
• Patients presenting a pregnancy or breast-feeding
• High sensibility to OT
• High sensibility to the excipients of the product
• Patients having family history of genetic pathology causing an extension of the interval QT
• Patients having risk factors of advanced twist