The effect of exogenous melatonin on libido and quality of buck semen is not investigated sufficiently as yet, whereas data regarding the effect of melatonin on ram fertility were often controversial. Thus, the aim of this study was to investigate the effect of exogenous melatonin on libido and standard sperm quality parameters in bucks out of breeding season. Twelve bucks of French alpine breed aging from 2-4 years were assigned into experimental and control group comprising 6 bucks each. Total duration of the experiment of 3 months from March to June was divided into 6 periods in a way that each months was divided into the first and second half. The experimental group of bucks received 4 melatonin implants (74 mg) at the end of March, while the control group of bucks was tested without melatonin implants. Semen samples were taken from all bucks by artificial vagina once per week and simultaneously their libido was estimated. In the obtained samples volume and concentration of semen, massive motility and motility of spermatozoa, proportion of live and pathologic forms, total number of spermatozoa and total functional spermatozoa fraction were calculated. The experimental group of buck had significantly higher values of massive motility and motility of spermatozoa in the first half of April as well as the proportion of live spermatozoa in the first and second half of April (P<0.05). The intensity of libido was higher in the experimental than in the control group of bucks during the experimental period, but differences were not significant (P>0.05). The results indicated that the application of exogenous melatonin significantly improved the qualitative parameters of semen in bucks as established by increased massive motility, motility of spermatozoa and the proportion of live spermatozoa. These beneficial effects of melatonin could be ascribed to its antioxidative effects on spermatozoa. However, this finding should be studied in more details by further analyses.