It is accepted that whilst hormones such as oxytocin, vasopressin and prostaglandin F2α induce myometrial contractions, essentially via an elevation of intracellular calcium, other ligands, such as β-adrenoceptor agonists, calcitonin gene-related peptide, and prostaglandin E2, promote uterine quiescence via their ability to increase intracellular cyclic AMP levels. At present, the exact factors initiating human parturition remain unknown, and labour may occur due to a loss of uterine quiescence, an increase in uterine contractility, or a combination of both. Whilst many studies have aimed to understand the mechanisms underlying uterine contractility there is a relative paucity of data regarding myometrial relaxation. We have verified the presence of mRNA encoding adenylyl cyclase (AC) isoforms I, II, III, V, VI, VII, VIII and IX in both non-pregnant and pregnant human myometrium, and in isolated myometrial cells maintained in cell culture. Furthermore, by means of immunoblotting and immunocytochemistry, we have demonstrated the expression of these isoforms as membrane-associated AC proteins, and identified changes in individual AC isoform expression during gestation. These findings illustrate the diversity of potential cAMP generating pathways in human myometrium, and the complexity of the signal transduction systems underlying uterine quiescence. Experimental Physiology (2001) 86.2, 265-272.