Hydrogen sulphide (H2S), similar to nitric oxide and carbon monoxide, is an endogenous gaseous signalling molecule, shown to play a prominent role in the regulation of contraction of many smooth muscles including myometrium1,2. H2S is predominantly produced from L-cysteine via the cystathionine γ-lyase and cystathionine β-synthase3. Both are expressed in uterus4. H2S and H2S-releasing compounds are potent inhibitors of rat and human myometrial contractions with evidence to suggest activities at the KATP channels, Cl- channels and on Ca entry1,2. H2S also exerts anti-inflammatory, anti-oxidant and cytoprotective actions. A better understanding of how uterine contractility is controlled is requierd in order to develop better tocolytics to reduce the morbidity and mortality associated with preterm delivery. Non-steroidal anti-inflammatory drugs (NSAIDs) e.g. indomethacin, which inhibit cyclo-oxygenase (COX) and production of prostaglandins, are effective in attenuating uterine contractions, but have limited use due to adverse fetal side-effects. ATB-346 is a NSAID, derived from naproxen but coupled to a H2S-releasing moiety, 4-hydroxy-thiobenzamide (TBZ)5. This study aimed to compare the effects ATB-346 with naproxen and TBZ alone and the H2S donor, Na2S, on human myometrial contractility. Isometric tension recordings were performed on strips of pregnant myometrium (1x5x2mm), from 15 women undergoing pre-labour Caesarean section (with informed consent and ethical approval). Firstly, the effect of increasing concentrations (0.3µM-100µM) of Na2S on force of contraction (mN) and contraction integral (area under the curve, AUC) was investigated (n=5). Next, the effect of ATB-346 (10µM and 30µM) was compared to the unconjugated naproxen or TZB alone (n=5-10). Significance was taken as P<0.05 by one-way ANOVA and Dunnett’s multiple comparison test. Acute application of Na2S to contracting strips of human myometrium produced a concentration-dependent decrease in activity. The decrease in force amplitude of contraction was significant at 30µM Na2S and for AUC at 100µM Na2S (P<0.01 and P<0.001 respectively). ATB-346 also produced a significant decrease in force amplitude and AUC which was observed at 10µM: 74.5(±8.4)% and 70.8(±5.8)% of control activity respectively (P<0.05). This effect was further potentiated at 30µM; 41.2(±14.0) % and 39.0(±11.3)% of control activity respectively (P<0.01). Naproxen and TZB alone had small but non-significant effects on contraction amplitude and AUC at both concentrations (P>0.05). Data confirm the tocolytic effect of H2S donors in human myometrium. The combination of COX inhibition together with the release of H2S resulted in a significant reduction of human myometrial contractility compared to Na2S alone. The more profound effect of ATB-346 compared to naproxen alone is likely due to the presence of H2S. Combination approaches involving H2S donors should be further explored as potential tocolytic therapies in myometrium.