The present study aimed to examine the antidepressant-like effect of the neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha, 5alpha THP) using the forced swim test in mice. Intracerebroventricular (ICV, 1 or 2 microg/mouse) or intraperitoneal (IP, 0.5, 1, or 2 mg/kg) administration of 3alpha, 5alpha THP dose-dependently reduced the duration of immobility in forced swim test without accompanying changes in ambulatory or rearing behaviors in the open-field test. The antidepressant-like effect of 3alpha, 5alpha THP (1 microg/mouse, ICV) was potentiated by prior administration of the GABA(A) receptor agonist, muscimol (0. 5 mg/kg, IP) and blocked by prior administration of GABA(A) receptor antagonist, bicuculline (1 mg/kg, IP). Administration of the agonist at diazepam binding inhibitor receptors, 4'-chlorodiazepam (4'CD, 15 mg/kg, IP) or N,N-di-n-hexyl-2-(4-fluorophenyl)-indol-3-acetamide (FGIN 1-27, 1 or 2 microg/mouse, ICV), the 11beta-hydroxylase inhibitor, metyrapone (150 mg/kg, IP and 1 or 2 microg/mouse, ICV) and the selective serotonin reuptake inhibitor (SSRI), fluoxetine (20 mg/kg, IP), which are known to increase the endogenous level of neurosteroids, also reduced the duration of immobility in forced swim test. The tricyclic antidepressant, imipramine (20 mg/kg, IP), which does not increase the 3alpha, 5alpha THP in the brain, also reduced the immobility time. While the antidepressant-like effect of fluoxetine, which is known to selectively increase the brain content of 3alpha, 5alpha THP, was either blocked partially by bicuculline (1 mg/kg, IP) or potentiated by muscimol (0.5 mg/kg, IP), the antidepressant-like effect of imipramine was not modified by bicuculline. These results demonstrate the antidepressant-like effect of the neurosteroid 3alpha, 5alpha THP, and suggest further evaluation for its development as a new class of antidepressant drug.