In the present work an attempt is made to get a deeper insight into the mechanism of labor and the events leading to the onset of labor by means of radioimmunological measurements of OT, PGE, PGF, PGEM and PGFM and by determining the oxytocin sensitivity and the concentration of oxytocin receptors. Prostaglandins play a major role for the mechanism of labor in labor of spontaneous onset as well as in several forms of induced labor (intravenous infusion of OT, amniotomy, local application of PGE2). The reason for this seems to be the 3 fold action of prostaglandins: stimulation of myometrial contractions, cervical softening, induction of gap junctions. Moreover prostaglandins produced in the placenta play a major role in the mechanism of placental separation and expulsion. Oxytocin seems to be of importance for the initiation of labor and the final expulsion of the fetus. Immediately before the onset of regular contractions a marked increase of oxytocin sensitivity can be demonstrated which correlates very well with an increase of oxytocin receptor concentration in the myometrium and decidua. Due to this increase in oxytocin sensitivity no rise in oxytocin plasma levels is necessary to induce labor. Apart from the induction of myometrial contractions oxytocin leads via receptors in the decidua to a stimulation of prostaglandin synthesis which can also be demonstrated in vitro. In cases of premature contractions the same mechanisms seem to be operational as at term, oxytocin and prostaglandins again playing a major role. Inhibition of contractions with ethanol is based on the capacity of alcohol to inhibit oxytocin secretion. The contractions inhibiting effect of ritodrine is mediated through the cAMP induced relaxation of the myometrium although possibly a direct reduction of prostaglandin synthesis by ritodrine is possible. Increasing estrogen and decreasing progesterone activities at term lead to multiple subtile changes leading to an increased prostaglandin synthesis and mainly to a rise in oxytocin receptor concentration in the myometrium and the decidua. Oxytocin from the fetal and maternal side stimulates contractions in the myometrium and prostaglandin synthesis in the decidua leading to the onset of labor. With progressing cervical dilatation prostaglandin synthesis is further stimulated; these prostaglandins together with the increased oxytocin plasma levels in the second stage of labor lead to expulsion of the fetus. After delivery prostaglandin synthesis in the placenta leads to placental separation and expulsion.(ABSTRACT TRUNCATED AT 400 WORDS)