Enkephalin analogs for multivalent ligand systems, bivalent enkephalins, multivalent enkephalins on polymers, multivalent ligands on vesicles, simultaneous activation of two different receptor systems, and cell interactions of enkephalin/polypeptide conjugates are described. Multivalent ligand systems can trigger receptor-receptor interactions and are considered to possess interesting possibilities in terms of enhanced potency, reduction of side effects, and a new biological activity.