The diagnosis of autism spectrum disorder (ASD) varies from very mild to severe social and cognitive impairments. We hypothesized that epigenetic subconvulsive activity in early postnatal life may contribute to the development of autistic behavior in a sexrelated manner. Low doses of kainic acid (KA) (25-100 µg) were administered to rat pups for 15 days beginning on postnatal (P) day 6 to chronically elevate neuronal activity. A battery of classical and novel behavioral tests was used, and sex differences were observed. Our novel open handling test revealed that ASD males nose poked more often and ASD females climbed and escaped more frequently with age. In the social interaction test, ASD males were less social than ASD females who were more anxious in handling and elevated plus maze (EPM) tasks. To evaluate group dynamics, sibling and non-sibling control and experimental animals explored 3 different shaped novel social environments. Control pups huddled quickly and more frequently in all environments whether they socialized with littermates or non-siblings compared to ASD groups. Non-sibling ASD pups were erratic and huddled in smaller groups. In the object recognition test, only ASD males spent less time with the novel object compared to control pups. Data suggest that chronic subconvulsive activity in early postnatal life leads to an ASD phenotype in the absence of cell death. Males were more susceptible to developing asocial behaviors and cognitive pathologies, whereas females were prone to higher levels of hyperactivity and anxiety, validating our postnatal ASD model apparent in the pre-juvenile period. Highlights• Chronic subconvulsive activity in early life leads to autism phenotypes.• Juvenile males were susceptible to asocial behaviors and cognitive pathologies.• Juvenile females were prone to hyperactivity and anxiety validating sex differences.• Non-siblings were erratic in groups irrespective of sex.• A postnatal epigenetic model may drug screen for milder forms of autism.

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