Proteins from pregnant rat uterine myometrium, previously incubated in the presence of (32P)orthophosphate, were analyzed by one- and two-dimensional polyacrylamide gel electrophoresis and subsequent autoradiography. Electrophoretic patterns of Coomassie blue-stained proteins in preterm and labor myometria were indistinguishable. However, radioactivity incorporated into tubulin in preterm myometrium was predominantly associated with the beta-subunit, whereas in labor, the alpha-subunit was labeled. Endogenous phosphorylated alpha- and beta-tubulins were clearly identified on two-dimensional gel electrophoretograms by apparent molecular weights, isoelectric points, co-migration with marker brain tubulin, and reactivity to specific antitubulin antibodies. The antibodies were incubated with nitrocellulose sheets onto which the myometrial proteins were electrophoretically transferred from the two-dimensional gels. The switching in phosphorylation of beta- to alpha-tubulin in the labor myometrium appears to be a manifestation of estrogen action. The labor-specific phosphorylation changes occurred in parallel with changes in serum estradiol/progesterone ratios. Indeed, estrogen administered to ovariectomized rats caused the appearance of phosphorylated alpha-tubulin, and this effect was inhibited by progesterone. Although the significance of tubulin phosphorylation is not currently understood, the switching in phosphorylation from beta- to alpha-tubulin under estrogen domination may help clarify the role of microtubule phosphorylation.