Nemifitide is a novel pentapeptide antidepressant, which appears to be effective in the treatment of major depressive disorder (MDD). In the present study 81 patients with MDD, DSM-IV criteria were randomized following a 1-wk screening period to receive 30 mg/d nemifitide, 45 mg/d nemifitide or placebo in a 6-wk double-blind, multicentre, outpatient efficacy study. Nemifitide or placebo was delivered by subcutaneous injection for 2 wk daily for 5 days (Monday to Friday) in the first 2 wk and patients were followed up for a further 4 wk. The primary efficacy measure was the change from baseline on the Montgomery-Asberg Depression Rating Scale. Secondary measures included the 17-item Hamilton Psychiatric Rating Scale for Depression (HAMD), the CGI severity and improvement scale and the Carroll Self-Rating Scale for Depression. This proof-of-principle study demonstrated a statistically significant superiority of the 45-mg/d dose vs. placebo at the time-point of peak effect (1 wk after the end of treatment). There appeared to be a greater effect with the 45 mg/d nemifitide dose than with 30 mg/d. An additional exploratory analysis by stratification of all patients by severity above and below or equal to the median baseline HAMD score of 22 showed a higher percentage of responders for both doses of nemifitide with statistical separation from placebo for patients with baseline HAMD score of >22 (above the median). There was no significant difference among treatment groups for patients with baseline HAMD score of <or=22. Nemifitide showed a good tolerability and safety profile. There were no dropouts due to adverse events, and the incidence of side-effects with nemifitide was comparable with that of placebo.