Maprotiline-d3 is intended for use as an internal standard for the quantification of maprotiline by GC- or LC-MS. Maprotiline is a tricyclic antidepressant. It binds to the norepinephrine transporter (NET) and is selective for NET over the serotonin and dopamine transporters. Maprotiline also binds to the serotonin (5-HT) receptor subtype 5-HT2A, as well as histamine H1, muscarinic acetylcholine, α1-adrenergic, and dopamine D2 receptors. In vivo, maprotiline inhibits norepinephrine reuptake in rat brain and peripheral tissues. It reduces isolation-induced aggressive behavior and inhibits electrical foot-shock stimulation-induced belligerence in mice when administered at doses ranging from 3 to 10 mg/kg. Maprotiline also reduces aggressive behavior in rhesus monkeys housed in groups. Formulations containing maprotiline have been used in the treatment of depression and anxiety.