One of the roles previously reported for protein kinase C (PKC) is modulation of the activity of the phosphatidylinositol signaling pathway. Studies were performed to test the hypothesis that activation of PKC results in inhibition of agonist-stimulated phasic myometrial contractions: contractions that appear to be mediated by phosphatidylinositol signaling mechanisms comparable to those producing cytosolic calcium oscillations. In vitro isometric contraction studies were performed using myometrium from adult Sprague-Dawley rats. Oxytocin and aluminium fluoride (a G-protein activator) produced comparable increases in phasic contractile activity. Phorbol 12,13-dibutyrate (PDB) significantly suppressed agonist-stimulated phasic myometrial contractions; in contrast, phorbol 13,20-diacetate (PDA), an inactive phorbol ester, had no significant effect on myometrial contractions. Prolonged exposure of myometrial tissue to PDB failed to down-regulate myometrial PKC and had no consistent effect on spontaneous and oxytocin-stimulated phasic contractions. These studies have provided support for a role for PKC as an intracellular regulator of the phosphatidylinositol signaling pathway, which itself appears to be part of the myometrial calcium oscillator that results in agonist-stimulated phasic myometrial contractions.