There is evidence that nitric oxide (NO) donor compounds have a relaxant effect on myometrial contractions. It has been reported that nitric oxide (NO) substrate has a relaxant effect in rat and human myometrium and that NO synthase inhibitors increase myometrial contractile activity. The objective of these experiments was to investigate the effects and potency of the NO donor compounds GSNO (S-nitroso-L-glutathione), SNAP (S-nitroso-N-acetylpenicillamine), SIN-1 (3-morpholinosydnonimine) and GTN (glyceryl trinitrate), the NO substrate L-arginine (L-arg) and the NO synthase inhibitors L-NAME (N G -nitro-L-arginine-methyl-ester) and L-NOARG (L-nitro-L-arginine) on the spontaneous and agonist-induced contractile activity of isolated human pregnant myometrium. Biopsies of myometrium were obtained at caesarean section with written consent and approval of the UCLH Ethical Committee. Strips were mounted for isometric recording under 2g tension in Krebs solution at 37 0 C and gassed with 95:5% O 2 :CO 2 . After equilibration (2h) the contractile activity of spontaneous contractions, or those elicited by 1nmol/L oxytocin, was measured for a 30-minute period using Grass FT.03 transducers and the data acquisition system Inc.MP100WS (Biopac Systems). GSNO, SNAP, SIN-1 or GTN (10 - 8 - 10 - 3 M) were then added to the tissue bath in a cumulative fashion. L-arg (10 - 8 - 10 - 3 M), L-NAME (10 - 5 - 10 - 3 M) and L-NOARG (10 - 5 M) were applied to strips with spontaneous contractions only. Contractile activity was assessed by measuring total integrated tension, the maximum tension achieved and the average tension per contraction, for each 30-minute period. GSNO, SNAP, SIN-1 and GTN all inhibited spontaneous (n= 5-9) and oxytocin-induced (n=4-8) myometrial contractions in a dose dependent fashion achieving maximal inhibition. The -log EC 5 0 (mean+/-sem) values for spontaneous and oxytocin -i induced contractions varied from 4.44 +/- 0.38 to 7.21+/- 0.69 with significant variation in potency between the NO donor compounds. The compounds were approximately 10-fold more potent in reducing average tension per contraction than the other parameters measured. Intrinsic myometrial contractions were unaltered by the addition of L-arg, L-NAME or L-NOARG (n=5-8). These findings are at variance with earlier reported findings of the effects of NO substrate and NO synthase inhibitor in myometrial tissue. These results have implications for the relative potencies of different NO donor compounds for clinical use in preterm labor.