Elucidation of the molecular details of signal transduction through G-protein coupled receptors (GPCRs) awaits the solution of high-resolution structures of the receptor species involved in passing the extracellular information across the plasma membrane. The critical challenge in this effort is the production of sufficient quantities of active and homogeneous receptor species amenable to crystallization screening. We describe here the high-level expression in mammalian cells and characterization of a fusion complex between the kappa opioid receptor and its cognate G-protein alpha subunit, G alpha(i1). Optimization of growth conditions resulted in the highest level of active binding sites reported to date for either opioid receptors or GPCR-G alpha fusions. In cells, the kappa opioid receptor was stabilized against proteolysis in the context of the fusion protein and was competent to bind both agonists and antagonists. Coupling of the kappa opioid receptor with the G alpha subunit was demonstrated by changes in agonist affinity in the presence of guanine nucleotides and by agonist-induced increases in the rate of guanine nucleotide hydrolysis. In addition to representing a physiologically relevant signaling complex, the additional hydrophilic surface area provided by the G-protein may enhance the chances of producing well-diffracting crystals from the purified complex.