During hypertonic saline—induced abortions, intrauterine pressure (IUP) and plasma progesterone (P) have been measured and the uterine volume (V) estimated in 13 patients. During the initial third of the instillation-abortion time, the decrease in P was proportionately greater than the increase in IUP, indicating that P withdrawal precedes the evolution of IUP and clinical progress. In 2 additional patients, with low P levels, the evolution of IUP and abortion have been induced by a mere increase in V, using isotonic (rather than hypertonic) saline. In these 2 patients P withdrawal only manifested when the evolution of IUP approached completion. Since the changes in P did not account fully for the changes in IUP, the ratio V/P was calculated at various phases of hypertonic saline—induced abortion. It then became apparent that whenever V increases or P decreases markedly, without compensatory changes in P or V, the IUP evolves and pregnancy terminates. It is concluded that current concepts in myometrial regulation may be advanced by quantitating V and P in individual patients whose IUP is measured.