Earlier studies have implicated brain-derived neurotrophic factor in stress and in the mechanism of action of antidepressants. It has been shown that antidepressants upregulate, whereas corticosterone downregulates, brain-derived neurotrophic factor expression in rat brain. Whether various classes of antidepressants reverse corticosterone-mediated downregulation of brain-derived neurotrophic factor is unclear. Also not known is how antidepressants or corticosterone regulates brain-derived neurotrophic factor expression. To clarify this, we examined the effects of various classes of antidepressants and corticosterone, alone and in combination, on the mRNA expression of total brain-derived neurotrophic factor and of individual brain-derived neurotrophic factor exons, in rat brain. Normal or corticosterone pellet-implanted (100 mg, 21 days) rats were injected with different classes of antidepressants, fluoxetine, desipramine, or phenelzine, intraperitoneally for 21 days and killed 2 h after the last injection. mRNA expression of total brain-derived neurotrophic factor and of exons I-IV was measured in frontal cortex and hippocampus. Given to normal rats, fluoxetine increased total brain-derived neurotrophic factor mRNA only in hippocampus, whereas desipramine or phenelzine increased brain-derived neurotrophic factor mRNA in both frontal cortex and hippocampus. When specific exons were examined, desipramine increased expression of exons I and III in both brain areas, whereas phenelzine increased exon I in both frontal cortex and hippocampus but exon IV only in hippocampus. On the other hand, fluoxetine increased only exon II in hippocampus. Corticosterone treatment of normal rats decreased expression of total brain-derived neurotrophic factor mRNA in both brain areas, specifically decreasing exons II and IV. Treatment with desipramine or phenelzine of corticosterone pellet-implanted rats reversed the corticosterone-induced decrease in total brain-derived neurotrophic factor expression in both brain areas; however, fluoxetine reversed the decrease only partially in hippocampus. Interestingly, antidepressant treatment of corticosterone pellet-implanted rats increased only those specific exons that are increased during treatment of normal rats with each particular antidepressant. We found that although corticosterone and antidepressants both modulate brain-derived neurotrophic factor expression, and antidepressants reverse the corticosterone-induced brain-derived neurotrophic factor decrease, antidepressants and corticosterone differ in how they regulate the expression of brain-derived neurotrophic factor exon(s). © 2006 IBRO.