Rationale Acute tryptophan depletion (ATD) transiently lowers central serotonin levels and can induce depressive mood states and cognitive defects. Previous studies have shown that ATD impairs object recognition in rats. Objectives As individual differences exist in central serotonin neurotransmission, the impact of ATD may vary accordingly. In this experiment, we investigated the hypothesis that male serotonin transporter knockout (SERT-/-), rats marked by a lower SERT function, are more vulnerable to the effects of ATD in an object recognition task than male wildtype (SERT+/+) and heterozygous (SERT+/-) rats. Materials and methods Twelve male SERT+/+, SERT+-, and SERT-/- rats were treated with standard dose and low-dose ATD using a gelatine-based protein-carbohydrate mixture lacking tryptophan. In the control treatment, L-tryptophan was added to the mixture. Four hours after treatment, the rats were subjected to the object recognition task. In addition, the effects of ATD on plasma amino acid concentrations were measured, and concentrations of 5-HT and 5-HIAA were measured in the frontal cortex and hippocampus of these rats. Results Plasma TRP levels and central 5-HT and 5-HIAA levels were decreased in all genotypes after ATD, but effects were stronger in SERT-/- rats. The standard dose of ATD impaired object recognition in all genotypes. SERT-/- and SERT+/- rats were more vulnerable to low dose of ATD in the object recognition task compared to SERT+/+ rats. Conclusions These results indicate a greater sensitivity to ATD in SERT-/- and SERT+/- rats, which may be related to stronger central depletion effects in these rats.