Cross-regulations between G s and G i mediated pathways controlling the adenylyl cyclase activity have been clearly demonstrated in vitro. To elucidate whether activation of the -adrenergic pathway in the pregnant myometrium might affect G i proteins and 2 -adrenergic receptors (ARs), we treated late pregnant rats from day 18 to day 21 with twice-daily administration of isoproterenol (8 mg/ kg). This treatment increased myometrial cAMP levels and led after 76 h to a significant and maximal rise in the immunoreactive amount of myometrial G i 2 and G i 3 proteins (1·4-and 1·7-fold respectively) associated with a parallel increase of the steady-state levels of both G i 2 and G i 3 mRNA (1·6-and 1·9-fold respectively). Propranolol antagonized this response indicating the implication of the -adrenergic pathway. Nuclear run-on assays demonstrated that isoproterenol enhanced respectively by 1·3-and 1·2-fold the transcription rate of the G i 2 and G i 3 genes. Quantification of myometrial 2 -ARs by [3 H]rauwolscine binding revealed that the total number of receptors was also increased at 76 h by 1·7-fold when compared with controls, with no change in the affinity of the 2 -ARs for the ligand. This effect was antagonized by propranolol. Quantification of both 2A -and 2B -subtypes by Northern blotting analysis demonstrated that this elevation was due to a selective increase of the 2A -subtype mRNAs. The present results indicate that in vivo stimulation of the -adrenergic pathway by isoproterenol increases both G i 2 /G i 3 and 2A -AR expression in the pregnant rat myometrium. The possible contribution of such a mechanism in pregnancy-related changes of both entities is discussed.