The antidepressant effect of the Ac-D-Met-Pro-Arg-Gly tetrapeptide (Ac-D-MPRG), an analogue of the C-terminal fragment of arginine-vasopressin, has been studied in the forced swimming test on rats of both sexes at an age of 70 days. The tetrapeptide was chronically intranasally administered at dose of 0.01 μg/kg from day 8 to day 14 and from day 3 to day 21 after birth or in a single dose 5 min prior to testing. We observed changes in the behavioral parameters affected by tetrapeptide. The most pronounced effect of Ac-D-MPRG was observed during chronic administration from day 8 to day 14 after birth. The duration of passive swimming significantly increased, and the immobilization time sharply decreased. These data indicate the absence of a state of behavioral depression. Our results suggested that exogenous administration of the arginine-vasopressin derivative exhibited a prolonged positive effect on animal behavior if it is performed at the most important period in the formation of the vasopressinergic system, namely, the second week of postnatal development.