Effect of some prostaglandin synthesis inhibitors on the antinociceptive action of morphine in albino rats.

1. Five prostaglandin synthesis inhibitors, indomethacin, diclofenac, mefenamic acid, ibufen and paracetamol, were found to inhibit the antinociceptive action of morphine without affecting the antinociceptive response of an equi-analgesic dose of prostaglandin E1. 2. Diclofenac was found to be the most potent and longest acting, whereas paracetamol was the least potent and shortest acting. Indomethacin, mefenamic acid and ibufen occupied an intermediate position, both in potency and duration of action. 3. The results suggest that prostaglandins may be involved in the antinociceptive action of morphine in albino rats.