Oxytocin (OT) is a potent uterine agonist. Its receptor (OTR) is a G-protein coupled receptor that is down regulated by prolonged exposure to OT. We hypothesized that activation of protein kinase C (PKC) mediated this OT-induced decrease in OTR expression. Diminished PKC activity in late pregnancy could underlie the increase expression of uterine OTR preceding labor onset. Using cell cultures of transformed human uterine myocytes, we determined the effects of PKC agonists and antagonists on the expression of OTR. We also explored the effects of over-expression of activator protein-1 (AP-1; a mediator of many PKC and phorbol ester-induced effects), using adenoviral expression vectors for the AP-1 subunits c-Jun and c-Fos. Stimulation of PKC using the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) caused a rapid, significant (P ? 0.05) increase in c-Jun and c-Fos concentrations but a significant decrease in mRNA for OTR within 6 h, followed by a significant decrease in OT-binding by 24 h. Adenoviral infection of the cells with expression vectors for c-Jun and c-Fos increased the AP-1 subunits but had no effect on OTR expression. Further, there were no changes in c-Fos or c-Jun levels in human intrauterine tissues around the time of labor onset, as measured by western analyses. We conclude that phorbol ester treatment decreases OTR expression, likely through a mechanism that does not involve AP-1.