Serotonin is a neural transmitter involved in a variety of functions. Many pediatric neurologists and neural psychiatrists routinely put children with autism on Selective Serotonin Reuptake Inhibitors (SSRI's). The net effect of these drugs is to increase the availability of serotonin at the synapse because theoretically children have low serotonin levels. The SSRI's were originally developed to treat depression and include drugs like Prozac and Zoloft, which are well known. Some animal studies, using mice models of autism have confirmed that these drugs do indeed increase serotonin and alleviate some of the symptoms of autism. But any clinician who regularly treats autism will tell you that they do not always work. It seems to me that the SSRI's work about half the time in children and are well worth a trial. But why don't they work all of the time? In my view, it is because we may very well be dealing with different neurological mechanisms. We put kids who meet certain diagnostic criteria on the spectrum, but since there are no known biological markers for autism we do not know that the same neurological mechanism are the same.