Repeat breeding (RB) is one of the major problems that affect the reproductive efficiency and economy of milk production in dairy animals. So far, the etiopathogenesis of this pathology has not been defined completely. Stress has been hypothesized to be a cause of impaired reproductive efficiency. Stress may cause an overproduction of beta-endorphins and free radicals; in particular, reactive oxygen species (ROS). The aim of this work is to determine the concentrations of these substances in RB cows and to evaluate the correlation with the serum level of progesterone. The study was performed on 60 dairy cows: 26 RB and 34 control cows. Blood samples were collected on day 12 and day 16, after artificial insemination (AI) in all subjects, in order to assess the concentrations of progesterone, free radicals and beta-endorphins. The stressors, free radicals and beta-endorphins, that we considered, were higher in repeat breeders (day 12, 93.32(+/-1.91) UCarr and 0.50(+/-0.03) ng/ml; day 16, 94.42(+/-1.91) UCarr and 0.61(+/-0.03) ng/ml), with a lower level of progesterone, which probably is responsible for failure to conceive. The stress factors (free radicals and beta-endorphins) may actually enhance each other and induce an inhibition of progesterone synthesis in repeat breeders.