The aim of (his study was to reveal the occunence of nitric oxide (NO) concentration changes after synchronisation, cycle stages in cows. For this, the following were carried out respectively: Fourty Holstein cows were used to study and estrous cycles were synchronisated by using single doze PGF2a in norma) ovarial structure animals. Blood samples were taken Irom estrous positive and negative animals before and after synchronisation and analyzed for NO and progesterone. Blood samples were also taken from estrous positive animals during cycle stages and analyzed for NO. The response of synchronisation was negative in animals that had higher NO concentrations before synchronisation. Decrease of NO concentration after synchronisation was lower in estrous negative animals. The progesterone levels decreased in these animals. Different NO concentrations were determined in cycle stages. It was concluded that cases of subestrous in animals with higher NO could be observed and that the physiologic event that came into cycle stages could be effected by NO concentration in cows.