Uptake and incorporation of 3H-valine into soluble protein of cultivated astroglial cells from newborn rat cerebral hemispheres increased after incubating the cells for 4 hr in 10(-6) or 10(-5) M morphine. The effects, inhibited by high naloxone concentrations, were not mediated by cyclic adenosine monophosphate (cAMP)-activated binding sites for morphine or D-alanine2-methionine-enkephalin-amide (D-ala2-met-enk-amide). The cultures consisted of 60-70% astrogliallike cells. By changing the kinetics of labeling it was shown that morphine affects protein synthesis of the cells in a concentration- and time-dependent way. Even in the incubation medium increased amounts of labeled proteins were found in a time-dependent way after morphine administration in different doses. Gel electrophoresis in sodium-dodecyl sulphate (SDS) revealed that proteins with subunits in molecular weight (m.w.) regions of 40,000-50,000 were increased in the cells and in the medium after morphine (10(-6) or 10(-5) M) treatment. Labeled proteins with subunits of approximately 40,000 and approximately 70,000 in the cells and medium, respectively, were decreased after morphine treatment. By exploring the field of glial protein metabolism after morphine treatment, information could be obtained on the interactions between astroglia and neurons in opiate action and during tolerance development.