Numerous studies show significantly reduced sows fertility in the warm period of the year. The aim of this study was to investigate the effect of sows treatment with eCG preparation, 24h after weaning, on the estrous reaction level in the cool and warm season of the year. A significantly (p<0.01) higher number of sows, treated in warm season with single eCG injection of 1,000IU (primiparous) or 1,500IU (older sows), manifested estrus within 7 days after weaning (83.7%) in comparison with the sows which were not treated (63.9%). In addition, the treatment with eCG results in a high level of estrus synchronization, both in the warmer and in the cooler season of the year. Namely, on day 4 and day 5 after weaning, estrus was detected in 81% of eCG treated sows within the cooler season, and in 78% of sows treated in the warmer season. These values were significantly (p<0.01) lower in the control (untreated) sows (41.1% in the cooler, vs. 33.3% in the warmer season). The obtained results show that the treatment with placental gonadotropin can be an effective method of increasing sow fertility in the wormer season of the year.