Selective serotonin and noradrenalin reuptake inhibitors such as fluoxetine and desipramine, respectively, are efficacious in the treatment of depression and chronic stress. Although they inhibit the reuptake of the biogenic monoamines soon after administration, therapeutic improvements occur only after 2 or 3 weeks. Freezing response and potentiated startle are common responses to moderate fear contextual conditioning. However, freezing but not startle is increased in rats that undergo intense fear conditioning. In this study, we evaluated the effects of acute and subchronic administration of fluoxetine and desipramine on these responses in testing sessions, as indices of fear in moderate and high fear conditioning. Fluoxetine did not show any significant effect on the moderate fear conditioning but reduced freezing and restored the startle response in rats under intense fear conditioning. In comparison, desipramine had no effect on the startle response when administered acutely or subchronically while freezing of the intense fear conditioning was reduced. Our findings indicate that intense contextual fear conditioning is sensitive to subchronic treatment with fluoxetine and resistant to desipramine. Fluoxetine appears to restore the serotoninergic function in brain areas recruited by intense contextual fear conditioning. These effects of fluoxetine may underlie its reported efficacy in the pharmacotherapy of panic disorders.