Flumezapine-d8 (LY 120363-d8) hydrochloride is deuterated labeled Flumezapine hydrochloride. Flumezapine hydrochloride is a potent and balanced antagonist of the dopamine D2 receptor and the 5-hydroxytryptamine receptor (5-HT receptor). Flumezapine hydrochloride does not alter the increase in serum cortisol caused by kappa-opioid receptor agonists. Flumezapine hydrochloride inhibits the conditioned avoidance response in rats and has a low risk of extrapyramidal side effects. Flumezapine hydrochloride can be used in antipsychotic research.