The effect of morphine on the expression on COX-2 and iNOS was considered by evaluating: a) the effects of COX1 and COX2 inhibitors on morphine withdrawal in vitro. Tolmetin (selective COX-1 inhibitor) and meloxicam (selective COX-2 inhibitor) treatment before or after morphine were able of both preventing and reversing the naloxone-induced contracture after exposure to morphine in a concentration-dependent fashion. b) the role of NF-kB in the expression of morphine withdrawal by using the PDTC, an inhibitor of NF-kB activation. PDTC was able to reduce the naloxone-induced contracture after exposure to the morphine in a concentrationdependent fashion. c) the role of NO in the expression of morphine withdrawal. L-NAME was able dose dependently to reduce the naloxone-induced contraction after exposure to morphine whereas D-NAME at the same concentrations did not affect it. The inhibitory effect of L-NAME on morphine withdrawal was dose dependently reversed by L-arginine not by Darginine. Finally, GTN on its own significantly increased the naloxone-induced contraction after exposure to morphine and it was also able to reverse the inhibition of morphine withdrawal caused by L-NAME. d) the effect of morphine on COX-2 protein expression in LPS-stimulated J774 macrophages. Treatment of J774 macrophages with LPS caused an accumulation of PGs. The addition of morphine to the cells 30 min before LPS challenge increased significantly PGs production. The COX-2 immunofluorescence study indicated that the control cells showed only background staining, cell stimulated with LPS alone revealed a diffuse accumulation of COX-2 immunostaining in the cytoplasm: this accumulation of COX-2 immunostaining increase significantly in cells stimulated with LPS+Morphine. This effect was reverted by naloxone. e) the effect of morphine on NO production by LPS-stimulated J774 macrophages. Treatment of J774 macrophages with LPS caused an significant increase of NO Morphine added to the cells 0.5 h before activation with LPS, further increased NO production. This effect was reverted by naloxone. Morphine was not able to increase NO production when added after LPS challenge. The present paper provides a strong evidence that both PGs and NO are involved in the development of morphine withdrawal further indicating that during morphine withdrawal the opioid induces the expression of the COX-2 and iNOS enzymes.