Literature has suggested a possible association between the mu opioid and D1 dopamine receptors. Using BRET and a NLS-based assay, the mu opioid and D1 dopamine receptors were shown to specifically interact through the formation of a stable heterooligomer. Competition binding demonstrated no considerable changes in the binding affinities of either receptor following co-expression. However, single-point saturation binding displayed a significant increase in cell surface expression of the muOR upon co-expression with D1R, indicating novel regulation of receptor trafficking. Additionally, binding indicated a reduction in cell surface expression of the muOR upon co-expression with D5R, suggesting that these receptors may also heterooligomerize. A chimeric receptor approach was also utilized and suggests that select regions of the D1R are required for promoting muOR cell surface expression. Furthermore, immunocytochemistry demonstrated that these receptors co-localize in neurons of rat brain, supporting the possibility that these receptors may come into direct contact in vivo.