Nemifitide is a peptide analog of melanocyte-inhibiting factor (MIF) that has been reported to relieve depressive symptoms in a very short period. Tetragenex (formerly Innapharma) was developing nemifitide with potential in the treatment of depression. Clinical data obtained from two pilot clinical studies, double-blind, placebo-controlled, parallel-design, in-patient and outpatient, suggested that nemifitide may have the following advantages over current therapies: rapid onset of action (3–5 d), short period of administration (5–10 doses over a 1–2 wk period), and a relatively long duration of treatment effect (3–6 months). The mechanism of action of nemifitide was investigated by evaluating its in-vitro receptor binding and in-vivo localization in the rat brain as well as its interactions with psychoprobes indicative of specific neurotransmitter pathways in acute studies in rats. The results of these studies indicate that nemifitide readily crosses the blood–brain barrier and is localized mainly in the amygdala, hippocampus and frontal cortex of the rat brain. Its in-vitro binding to several receptors including 5-HT-2A, as well as its significant interaction with d-fenfluramine in the rat, suggest the participation of the serotonergic pathway in its mode of action in a different way than that observed with SSRI antidepressants.