Objectives: To investigate the effect of glyceryl trinitrate on isolated human pregnant uterine strips and whether the uterine relaxation induced by glyceryl trinitrate could be reversed by oxytocics used in current clinical practice. Design: In vitro pharmacological study. Setting: Department of Obstetrics & Gynaecology, National University of Singapore, National University Hospital. Participants: Eighteen women who delivered by caesarean section at term. Methods: Myometrial strips were preloaded with an initial tension of 1.5g in organ baths containing Krebs-Henseleit solution which was aerated with oxygen in 5% carbon dioxide and maintained at 37°C, pH 7.4. The effect of glyceryl trinitrate was studied in strips displaying regular spontaneous contractions. The ability of oxytocin, ergometrine or prostaglandin F2α to stimulate uterine contractions was assessed in strips where uterine activity was significantly inhibited by glyceryl trinitrate. Results: Glyceryl trinitrate reduced the amplitude and frequency of spontaneous uterine contractions in a concentration-dependent manner, although the sensitivity of the myometrial strips varied considerably from one specimen to another. The concentration of glyceryl trinitrate producing complete inhibition of myometrial contractions ranged from 44-705μM. In the presence of glyceryl trinitrate which markedly depressed spontaneous contractions, oxytocin (20mU/mL), ergometrine (6.15μM) and prostaglandin F2α (6.15μM) were capable of reversing the uterine activity to either higher than or the untreated level of contractility. Conclusions: This study demonstrates that glyceryl trinitrate is a potent uterine relaxant in vitro and that the tocolytic effect could be reversed with ease by oxytocics.