Although opiates significantly alter RNA and protein synthesis in a variety of neuronal cell types, their effect on the biosynthetic activity of regenerating neurons has not been investigated. In the present study, the effect of morphine on the incorporation of [3H]L-lysine into proteins of facial nucleus neurons was examined by light microscopic radioautography. Silver grains present within various compartments of normal and regenerating (3-, 7-, 14- and 21 days post-axotomy) neurons from saline-treated Wistar rats were compared with the amount present in similar cells from animals receiving 40 mg/kg morphine sulfate i.v. At 14- and 21-days post-axotomy, regenerating neurons were larger and the grain count in the emulsion over these cells was greater than that observed in normal (unoperated) neurons. In normal facial neurons, the accumulation of lysine into the nucleus and nucleolus was significantly lower 60 min after morphine administration. However, morphine's inhibition of lysine incorporation was even more pronounced in regenerating neurons. In these cells, nuclear lysine uptake was depressed at 3 and 7 days, while maximum inhibition of cytoplasmic incorporation occurred at 14-days post-axotomy. Morphine administration decreased nucleolar lysine incorporation at all survival intervals.