Patients taking atypical antipsychotics (AAPs), especially clozapine, are often associated with hyperglycaemia. Here, clozapine served as a representative agent for investigating how AAPs induce hyperglycaemia. In normal mice and mice fed a high fat diet (HFD), clozapine impaired glucose tolerance and glucose-stimulated insulin secretion (GSIS) following intraperitoneal glucose administration and increased plasma 5-HT levels. Intraperitoneal 5-HT administration also impaired glucose tolerance and GSIS in mice. In INS-1 cells, high 5-HT levels impaired GSIS, which was attenuated by the 5-HTR