HPC-1/syntaxin 1A (STX1A) is thought to regulate the exocytosis of synaptic vesicles in neurons. In recent human genetic studies, STX1A has been implicated in neuropsychological disorders. To examine whether STX1A gene ablation is responsible for abnormal neuropsychological profiles observed in human psychiatric patients, we analysed the behavioral phenotype of STX1A knockout mice. Abnormal behavior was observed in both homozygotes (STX1A−/−) and heterozygotes (STX1A+/−) in a social interaction test, a novel object exploring test and a latent inhibition (LI) test, but not in a pre-pulse inhibition test. Interestingly, attenuation of LI, which is closely related to human schizotypic symptoms, was restored by administration of the selective serotonin reuptake inhibitor, fluoxetine, but not by the dopamine reuptake inhibitor, GBR12935, or the noradrenalin reuptake inhibitor, desipramine. We also observed that LI attenuation was restored by DOI (a 5-HT2A receptor agonist), but not by 8-OH-DPAT (a 5-HT1A receptor agonist), mCPP (a 5-HT2C receptor agonist), SKF 38393 (a D1 receptor agonist), quinpirole (a D2/D3 receptor agonist) or haloperidol (a D2/D3 receptor antagonist). Thus, attenuation of LI is mainly caused by disruption of 5-HT-ergic systems via 5-HT2A receptors. In addition, 5-HT release from hippocampal and hypothalamic slices was significantly reduced. Therefore, ablation of STX1A may cause disruption of 5-HT-ergic transmission and induce abnormal behavior.