The concept that the progesterone-estrogen ratio determines gap junction abundance in myometrium has developed from studies in laboratory animals. In the humans, however, gap junctions form in abundance at term despite a high progesterone-estrogen ratio. We have therefore examined the relationship between contractile activity, progesterone and estrogen levels and gap junction abundance in human myometrium at term. Myometrial tissue obtained from women who had an elective or emergency Cesarean operation at the 38th or 39th week of pregnancy was immunohistochemically examined using a polyclonal antiserum raised against a synthetic peptide matching part of the sequence of connexin43, the principal component of myometrial gap junctions. The concentration of progesterone and estradiol in the maternal blood was measured by means of radio-immunoassay. The number of gap junctions was estimated by counting of punctate staining patterns obtained in myometrial tissue as viewed by laser confocal microscopy and calculated per 1 microns 3 of tissue. The number of gap junctions measured was found to be related to the presence or absence of labor. In the myometrium of women in labor (active phase), the number of gap junctions was highest (45.25 +/- 35). When the myometrium was not contracting (non-labor) and when contractions were weak (pre-labor) the number of gap junctions was low (15.16 +/- 10-5.75 +/- 5). In the pre-and active phase of labor, the concentration of progesterone in maternal blood was significantly higher (P < 0.025 and 0.03, respectively) than in women who were not in labor. The present study shows the potential of using optical section series obtained by laser confocal microscopy for the quantification and mapping of the three-dimensional distribution of gap junctions in human myometrium at term.