Signs of autism have been found in fish swimming in water contaminated with psychoactive medications.
Scientists identified patterns of gene activity in the fathead minnows that mimicked those seen in susceptible humans with the developmental disorder.
The findings suggest a potential environmental trigger for autism in genetically vulnerable people, the study authors said.
Researchers from Idaho State University in the US exposed fish to the antidepressant Prozac (fluoxetine) and two other chemicals. These were venlafaxine, another antidepressant, and carbamazepine, used to control seizures.
Concentrations were comparable with the highest estimated environmental levels.
The genetic pathways affected were the same as those associated with "idiopathic" autism spectrum disorders whose cause is unknown.
Experts suspect the disorders were brought about by a combination of genetic and environmental factors.
The new findings are published in the online journal Public Library of Science ONE.
Lead scientist Dr Michael Thomas said: "While others have envisioned a causal role for psychotropic drugs in idiopathic autism, we were astonished to find evidence that this might occur at very low dosages, such as those found in aquatic systems."