A series of mu opioid receptor gene Oprm splice variants have been reported that differ only at their C‐terminus. These variants all contain exons 1, 2, and 3 of the gene, the exons responsible for coding all seven transmembrane domains. Whereas MOR‐1 also has exon 4 that encodes for an additional 12 amino acids at the tip of the C‐terminus, the other MOR‐1 variants have unique amino acid sequences distinct from those in MOR‐1 due to alternative splicing. All these variants are mu‐selective in binding assays. The current study explored the ability of these variants to stimulate [³⁵S]GTPγS binding to assess them functionally. Only mu opioids stimulated [³⁵S]GTPγS binding. Among the mu opioids we noted marked differences in their maximal stimulation among the clones. This was most prominent with β‐endorphin, which stimulated [³⁵S]GTPγS binding in the MOR‐1E expressing cells to a greater degree than [D‐Ala²,MePhe⁴,Gly(ol)⁵]enkephalin (DAMGO; 130%) and was far less effective than DAMGO in MOR‐1C cells (44%). The rank order of maximal stimulation of the drugs varied among the clones as well. Dynorphin A, β‐endorphin and morphine were most effective in stimulating [³⁵S]GTPγS binding in MOR‐1E, while M6G and fentanyl were most effective in MOR‐1 expressing cells. The potency (EC₅₀) of some of the drugs also varied extensively among the clones, with a poor correlation between the potency of the drugs to stimulate [³⁵S]GTPγS binding and their binding affinity. Together, these findings reveal marked functional differences among the variants that only can be explained by their structural differences at the tip of their C‐terminus. Synapse 51:11–18, 2004. © 2003 Wiley‐Liss, Inc.