Influence of the melatonin implantation on sperm quality, plasma testosterone levels and biochemical composition of the seminal plasma was evaluated in rams in the breeding and non-breeding season. Melatonin administration during the non-breeding season improved progressive motility and morphologically normal sperm rates. The mean plasma testosterone level was significantly higher in melatonin treated animals compared to untreated ones. Fructose, sodium, calcium, magnesium levels, and aspartate aminotransferase and lactate dehydrogenase activity in the seminal plasma were similar in melatonin treated and control rams. In contrast, melatonin administration during the breeding season did not enhance spermatologic parameters and plasma testosterone levels. Biochemical composition of the seminal plasma was also comparable in melatonin treated and untreated control groups.