Objective        To study the change of anxious behavior and secretion of adrenocorticotropic hormore (ACTH) and corticosteron in serotonin transporter (SERT) knockout mice.       Methods        Heterozygous SERT mutant mice (SERT

+/-

) were mated to produce offspring. Thirty offspring mice were divided into control group (n=15) and elevated plus maze (EPM) group (n=15) according their genotype (SERT

+/+

SERT

+/-

SERT

-/-

). The mice were stimulated with EPM as a stressor and their anxious behaviors were observed. ACTH and corticosteron levels in the mice were measured.       Results        (1) The genotypes were identified in SERT

+/+

SERT

+/-

and SERT

-/-

mice respectively by PCR. (2) The open arm time (OT)

the OT %

the open arm entry (OE) and OE% were significantly lower (P0.05

P0.01).(3) The ACTH level was significantly higher in SERT

+/-

and SERT

-/-

mice than in SERT+/+ mice after EPM stimulation (P0.05) while the corticosteron level was significantly higher in SERT

-/-

mice than in SERT

+/+

mice (P0.05).       Conclusion        SERT knockout mice are easy to have anxiety with their ACTH and corticosteron level significantly increased under stress.