Myometrial tissues from guinea pigs were quantitatively examined for gap junctions in electron micrographs. Small numbers of gap junctions were present between smooth muscle cells in myometria of pregnant guinea pigs at days 50 and 65 of gestation. The junctions increased in number and size at parturition on day 69 and decreased again to control levels 24 h after parturition. A similar increase in junctions occurred when abortion was induced by 16,16-dimethylprostaglandin E2 (PGE2) on day 65. There were no consistent or significant differences in numbers of gap junctions from myometrium taken over sites of placental attachment and from other sites. These results together with previous studies suggest that an increase in myometrial gap junction area is associated with and may be essential for parturition in guinea pigs, but the control of their development may differ from that in other mammals.