Objective: We intended to demonstrate the presence of L-arginine-nitric oxide system in rat myometrium and detect the possible mechanism of action of nitric oxide (NO) on rat myometrium during the last week of gestation.Study design: The effect of L-arginine-as a NO donor- was examined on the spontaneous and oxytocin as well as carbachol induced contractions of pregnant rat uterine strips. L-NAME was used as a nitric oxide synthase (NOS) inhibitor to block the obtained action of L-arginine. Lastly, the effect of methylene blue- as a soluble guanylate cyclase inhibitor - and glibenclamide- as a specific adenosine triphosphate-dependent potassium K+(ATP) channels- on the effect of NO donor on the spontaneous contractions of isolated pregnant rat uterine strips.Results: (1) 0.1, 0.2, 0.4, and 1.6 mg/ml L-arginine significantly inhibited both amplitude and frequency of spontaneous contractions of isolated pregnant rat uterine strips in a dose dependent manner.(2) L-arginine (0.4 mg/ml) antagonized the stimulatory action of oxytocin (1 mu/ml) and carbachol (0.1 ug/ml) significantly on the amplitude and the frequency of isolated pregnant rat uterine strip contractions. (3) L-NAME (0.01, 0.02, and 0.04 mg/ml) significantly blocked the inhibition-induced by 0.4 mg/ml L-arginine on both amplitude and frequency of spontaneous contractions of isolated pregnant rat uterine strips. 4) Methylene blue (0.04 and 0.08 mg/ml) and glibenclamide (0.2 and 0.4 mg/ml) produced significant partial inhibition of the effect of L-arginine on the uterine strips.