Persistent breeding-induced endometritis (PBIE) is a major cause of infertility in the mare. Delayed uterine clearance, attributed mostly to impaired myometrial contractility, is recognized as an important factor in the development of PBIE. Nitric oxide (NO) may play a role in determining susceptibility of mares to PBIE through an inhibitory effect on uterine contractility. The purpose of this research was to investigate the role of the nitric oxide system in the pathogenesis of PBIE in mares. First, a randomized controlled experiment was used to evaluate the effect of NO on spontaneous uterine contractility. Using an in-vitro muscle contractility model, uterine tissue strips from euthanized mares were treated with increasing concentrations of S-nitroso-N-acetylpenicillamine (an NO donor) and N-acetyl-D-penicillamine (vehicle and time-matched control) at regular intervals. Results indicated a dose-dependent inhibitory effect of NO on spontaneous uterine contractility. Secondly, differences in endometrial nitric oxide synthase (NOS) activity between PBIE-susceptible and resistant mares, and the effect of a specific inducible nitric oxide synthase (iNOS) inhibitor on endometrial NOS activity, were evaluated. Mares susceptible or resistant to PBIE were selected based on preset criteria and the results of an intrauterine challenge with killed spermatozoa during estrus. Endometrial biopsy samples were cultured in L-arginine supplemented minimum essential medium with or without a specific iNOS inhibitor. The medium and the endometrial tissue were collected after 24 hours of culture to determine endometrial NOS activity. Susceptible mares had significantly greater endometrial NOS activity than resistant mares. The iNOS inhibitor treatment resulted in a significant reduction in endometrial NOS activity. Coupled with the previously reported presence of increased NO levels in the uteri of susceptible mares, the dose-dependent inhibition of spontaneous uterine contractility by NO and the greater endometrial NOS activity in susceptible mares in the studies described in this thesis suggest that the nitric oxide system may play an important role in the development of PBIE in mares. The reduction of endometrial NOS activity by the iNOS inhibitor treatment provides a basis for clinical testing of NOS inhibitors as preventative or therapeutic options for PBIE in mares.