Rat and guinea pig fetal brain cell cultures and immunoblotting techniques were used to study the effect of receptor selective opioids on the level of the membrane-bound ai and a, GTP binding protein subunits. Incubation of rat hindbrain cultures with the p selective peptide DAGO decreased the amount of both a proteins. The reduction observed was equivalent to 36% in a, and 41% in ai. On the other hand, incubation of rat forebrain cultures with this peptide had an opposite effect, increasing the a, and ai levels by 66% and 68%, respectively. This differential effect of the peptide on the G proteins at the two brain areas may reflect the selective interaction at the receptor level; DAGO induced a fast and effective receptor down-regulation (50% decrease in B,,,) in hindbrain but not in forebrain cultures. Moreover, 6 and p selective ligands differed in their effect, as indicated by the finding that the 6 selective peptide DPDPE increased the amount of both a proteins in hindbrain cultures by 40%. Similar experiments conducted with guinea pig brain aggregate cultures indicated that the K selective agonist U50,488 decreased the amount of the membrane bound ai protein subunit by 56%. The results thus indicate that opioid agonists, interacting selectively with the three types of opioid receptors, induce a complex repertoire of changes in the immunoreactive levels of the membrane-bound a GTP binding protein subunits in various CNS structures.