The aim of this study was to determine whether chronic opioid treatment could influence the protein kinase C (PKC) activity in the rat brain. Chronic morphine (microns agonist) but not butorphanol (mu/delta/kappa mixed agonist) treatment enhanced cytosolic PKC activity in the pons/medulla, but not in the cytosolic fractions of cortex and midbrain regions. Concomitant administration of the opioid receptor antagonist, naloxone, blocked the PKC upregulation by chronic morphine. Chronic administration of morphine and butorphanol produced no change in the membrane PKC activity. Antinociceptive tolerance to morphine but not to butorphanol was developed under these conditions. These results suggest that chronic morphine administration leads to an upregulation of the cytosolic PKC activity in the pons/medulla through repeated activation of mu opioid receptors and that the PKC upregulation in this specific area may contribute to the morphine tolerance.