The objective of this study was to investigate the effects of Lactobacillus gasseri 505 (505) and the synbiotic combination of 505 and Cudrania tricuspidata leaf extracts (CT) on the hypothalamic-pituitary-gonadal axis in mice during chronic stress. In our previous study, a synbiotic combination (Syn) of a fermented CT-supplemented milk with 505 exhibited good anti-inflammatory and hepatoprotective activities in mice. Therefore, we hypothesized that a Syn may exert some preventive effects in the male reproductive system when suffering from unpredictable chronic mild stress (UCMS). UCMS significantly decreased the serum levels of corticosterone, however, treatment with 505 and its Syn suppressed UCMS-induced decreases. Histopathological analysis of the testes showed that these organs experienced some damage during UCMS, but this was repaired following treatment with 505 or Syn. Similarly, the transcription levels of gonadotropin-releasing hormone (GnRH), GnRH receptor, and gonadotropins, moreover, testicular development (i.e., Adam5, Adam29, and Spam1) - and steroidogenesis (i.e., Lhr, Egfr, and StAR) -related genes were significantly downregulated by UCMS. These UCMS-induced changes were inhibited following the administration of 505 or Syn, which was confirmed by the results of in situ hybridization analysis. These results suggest that the administration of a Syn could attenuate the testicular dysfunctions induced by UCMS.