Preterm premature rupture of the chorioamniotic membrane frequently leads to preterm birth and perinatal mortality. To ascertain whether the rupture of these membranes is influenced by variations in environmental pH and sodium concentration, we punctured 180 specimens from 9 membranes that were equilibrated in solutions of different pH, and 196 specimens from 10 membranes that were equilibrated in solutions with different sodium concentrations. Whole-membrane mechanical characteristics--strength, stiffness, toughness and ductility--were measured. These characteristics were defined based on a variant of the weakest-link theory: from a pool of three to five specimens, the specimen that had the lowest values of strength and concomitant values of stiffness, toughness, and ductility represents the mechanical characteristics of the entire membrane section. Strength-related mechanical characteristics--strength, stiffness and toughness--correlated negatively with pH (p < 0.001, p < 0.001, and p < 0.02, respectively), while ductility did not correlate significantly with pH. Membrane hydration and thickness correlated positively with pH (p < 0.001). The greatest increase in hydration accompanied by the greatest drop in strength, stiffness and toughness was observed between pH values of 3.68 and 5.58, suggesting that insufficient quantities of vaginal H รท could cause--as well as mark--premature rupture of the chorioamniotic membrane. No whole--membrane mechanical characteristics correlated significantly with changes in sodium concentration. Membrane hydration and thickness had slight positive correlations with sodium concentration (p < 0.05). Changes solely in the sodium concentration of the amniotic fluid during normal pregnancy are not sufficient to facilitate rupture at term.