Inclusion Criteria:
* Medically healthy outpatients between 6 and 17 years of age;
* Diagnostic and Statistical Manual 5th edition (DSM-5) criteria for Autism Spectrum Disorder (ASD) on the basis of clinical evaluation, confirmed with the Autism Diagnostic Interview Revised (ADI-R) and Autism Diagnostic Observation Schedule, Second Edition (ADOS-2) or Childhood Autism Rating Scale, Second Edition (CARS-2);
* males and females;
* intelligence quotient (IQ) of 40 and above;
* rating of 4 or higher on the Social Communication domain of the Clinical Global Impressions Severity (CGI-S);
* Social Responsiveness Scale-2 Total Score of 70 and above;
* care provider who can reliably bring participant to clinic visits, provide trustworthy ratings, and interacts with participant on a regular basis;
* stable concomitant psychotropic medications or medications potentially affecting vasopressin for at least 4 weeks (with the exception of fluoxetine, 6 weeks);
* no planned changes in psychosocial and biomedical interventions during the trial;
* willingness to provide blood samples and ability to participate in key study procedures (i.e., diagnostic assessments and laboratory safety measurements).
Exclusion Criteria:
* DSM-5 diagnosis of schizophrenia, schizoaffective disorder, or psychotic disorder;
* regular nasal obstruction or nosebleeds;
* unstable medical conditions such as migraine, asthma attacks, or seizures, and significant physical illness (e.g. serious liver disease, renal dysfunction, or cardiac pathology);
* clinically significant abnormal electrocardiogram reading;
* history of hypersensitivity to vasopressin, its analogs, or compounding preservatives (e.g., chlorobutanol);
* evidence of a genetic mutation known to cause ASD or intellectual disability (e.g., Fragile X Syndrome); or metabolic, or infectious etiology for ASD on the basis of medical history, neurologic history, and available tests for inborn errors of metabolism and chromosomal analysis;
* significant hearing or vision impairments;
* habitually drinks large volumes of water;
* pregnant or sexually active females not using a reliable method of contraception;
* current use of any medications known to interact with vasopressin including: 1) carbamazepine (i.e., Tegretol); chlorpropamide; clofibrate; urea; fludrocortisone; tricyclic antidepressants (all of which may potentiate the antidiuretic effect of vasopressin when used concurrently); 2) demeclocycline; norepinephrine; lithium; heparin; alcohol (all of which may decrease the antidiuretic effect of vasopressin when used concurrently); 3) ganglionic blocking agents including benzohexonium, chlorisondamine, pentamine (all of which may produce a marked increase in sensitivity to the pressor effects of vasopressin);
* previous participation in a vasopressin clinical trial or current use of vasopressin;
* current use of desmopressin (DDAVP) or oxytocin.
