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Effects of three peptides on male reproductive tract of beetle were analysed.All peptides increased soluble proteins concentrations in testes and their dry mass.Lepde-NPF-I influence on spermatocytes number and contractility of ejaculatory duct.Neb-TMOF decreased spermatocytes number but increased ejaculatory duct contractility.Neb-colloostatine increased spermatocytes number only in 8-day-old males.In insects, the majority of studies have been conducted on the hormonal regulation of female reproduction. Thus far, little is known about the regulation of male reproductive physiology, especially by peptide hormones. We report here, for the first time in insects, the effects of three peptides, Neb-colloostatin (SIVPLGLPVPIGPIVVGPR), Neb-TMOF (NPTNLH) and Lepde-NPF-I (ARGPQLRLRFa), on various aspects of reproduction in male Tenebrio molitor beetles.All three tested peptides increased the soluble protein concentration in the testes and the dry mass of the beetle's testes. They also significantly changed the protein profiles of the testes. Injection of these peptides also significantly changed the number of sperm cells in the testes. However, the observed effects were age specific. The most prominent changes were observed in 4-day-old males. Neb-colloostatin and Neb-TMOF decreased the number of sperm cells, whereas Lepde-NPF-I increased the number of spermatocytes. Moreover, in vitro experiments revealed that Neb-TMOF and Lepde-NPF-I increased the contractility of the ejaculatory duct of T. molitor males.The results obtained suggest that different reproductive processes in males might be regulated by complex mechanisms.