Abstract: The subjects of the current invention are compounds, which exhibit arginase inhibiting activity (including difluoromethylornithine (DFMO) and L-norvaline, but not limited to them), and which can be used as therapeutically active agents for the treatment and prevention of depression and/or depression-related conditions. Other subjects of the present invention are the use of said arginase inhibiting compounds as therapeutically active agents for the manufacture of pharmaceutical compositions for human and veterinary application, pharmaceutical composition comprising said arginase inhibiting compound and a method for treatment and prevention of depression and/or depression-related conditions. Also, a method for identifying compounds suitable for use as therapeutically active agents for treatment and/or prevention of depression and/or depression-related conditions is disclosed.