The enzyme tryptophan hydroxylase 2 (TPH2) catalyzes the hydroxylation of L-tryptophan to L-5-hydroxytryptophan (5-HTP), the first and the key step in 5-HT synthesis in the mammalian brain. Mutations in the human Tph2 gene reducing enzyme activity increase the risk of psychopathology. Pharmacological chaperones are small molecules that can specifically bind to mutant protein molecules, restore their disturbed 3D structure to the native state, and increase their stability and functional activity. The chaperone activity of (R)-2-amino-6-(1R,2S)-1,2-dihydroxypropyl)-5,6,7,8-tetrahydropterin-4(3H)-one (BH 4 ) is expressed by increasing the in vitro thermal stability of mutant tyrosine hydroxylase and phenylalanine hydroxylase molecules which are similar to TPH2 in their structure and characteristics. The P447R substitution in the mouse TPH2 molecule results in a 2-fold decrease in enzyme activity in their brains. We studied the effect of this mutation on the TPH2 thermal stability, as well as on the ability of BH 4 and its 8 structural analogues to increase the thermal stability of the mutant TPH2 from midbrain extracts of BALB/C mice. Temperature stability was studied by the decrease in enzyme activity during its heating for 2 min at increasing temperatures and was evaluated by the T 50 value that is the temperature at which the enzyme activity decreased by half. For the mutant TPH2, the T 50 value was decreased compared to the wild type enzyme. BH 4 and its closest structural analogue, 6-methyl-5,6,7,8-tetrahydropterin, increased the T 50 value, i.e. , exhibited chaperone activity. Other close BH 4 analogs, 6,7-dimethyl-5,6,7,8-tetrahydropterin and folic acid, were not effective. It can be assumed that BH 4 can be effective in the treatment of mental disorders caused by mutations in the Tph2 gene.