Whole blood serotonin and tryptophan were measured in 87 normal subjects and in 40 autistic subjects. Whole blood serotonin concentration (mean ± SE) were significantly higher in drug‐free (N= 21) autistics (205 ± 16 ng/ml) that in normal (136 ± 5.4 ng/ml) The Gaussian distribution of serotonin level of the unmidicated autistic group suggests the elevation was not due to a subgroup of autistic subject Autistics medicated with medicated with anticonvulsants or neuroleptics had significantly lower serortonin levels than did drug‐free autistic subjects Whole blood trytophan levels and platelet counts were similar in the autistic and normal groups. The possible cause of the hyerserotonemia of autism are discussed