This paper aims to study the seasonal reproductive activity mechanisms of sheep and explore what are the reproductive characteristics of Slovenian indigenous sheep breeds: the Jezersko-Solčava sheep, the Bovec sheep and the Istrian pramenka sheep. The majority of sheep breeds are polyoestrous meaning they only mate in a certain time frame of the year. The beginning and the duration of the mating seasons are regulated by several mechanisms, the most important being the photoperiod or day length. Sheep sense the change in the day length via their hypothalamus that regulates the secretion of the hormone melatonin from the pineal gland. Other important factors in the regulation of seasonal reproductive cycles are also air temperature and precipitation, especially in a tropical environment. The start of sexual maturity also depends on several interconnected factors such as birth season, herd size and breed, as well as nutrition and breeding environment. The research on the reproductive activity of Slovenian indigenous sheep breeds was carried out at the Infrastructure Centre for Sustainable Recultivation (ICSR) Vremščica of the Ljubljana University Veterinary Faculty. The research results were described in numerous scientific publications that have served as technical literature for this paper. In the paper we will describe at which age these sheep breeds enter sexual maturity, in which season they are reproductively active and, finally the course and the duration of the breading cycle.