Abstract:The calcium binding protein calmodulin and the opiate receptor binding sites are unevenly distributed in various subcellular fractions of neuroblastoma‐glioma NG108‐15 cells. The crude mitochondrial‐membrane fraction of these cells contains two membrane fractions that are separable by sucrose gradient centrifugation. These two differ in the content of both calmodulin and opiate receptors. Leucine enkephalin andd‐Ala2‐methionine enkephalinamide decrease the amount of membrane‐bound calmodulin in the NG108‐15 cells in a time‐ and dose‐dependent manner, whereas the opiate antagonists naloxone and levallorphan have an opposite effect. Naloxone blocks the effect of leucine enkephalin and dextrallorphan has no significant effect. The opiate alkaloids entorphine and phenazocine induce changes similar to that of the enkephalins whereas morphine is inactive even at high concentrations. The alteration in the amount of membrane‐bound calmodulin after a short incubation (15 min) with the enkephalins or with naloxone is reflected as an opposite change in the amount of calmodulin in the cell cytosol. Naloxone and levallorphan also increase the number of opiate receptors in NG108‐15 cells but dextrallorphan has no such effect. Modulation of the intracellular distribution of calmodulin by opioid peptides and alkaloids may control the activity of various membrane‐bound and cytosolic systems that are calmodulin‐