Three major opioid receptors, δ (DOR-1) (1,2), μ (MOR-1) (3–5), and κ (KOR-1) (6–9), and an opioid-like receptor (ORL-1/KOR-3) (10–16) have been identified by molecular cloning. Although each of the cloned opioid receptors is derived from a single gene, a number of alternatively spliced variants from their own genes have been isolated (16–20). One extraordinary example is the mouse μ opioid receptor (Oprm) gene in which alternative splicing of the fourteen exons generates at least 15 variants (21–25). It is difficult to study these cloned receptors in vivo. But expressing individual receptors in a particular cell line through transfection of the cloned receptor cDNAs offers a valuable system for exploring their pharmacological and biological properties, as well as their structure and function relationships. To successfully express the cloned receptors, several factors must be considered.