Endocytosis of the μ-type opioid receptor (MOR) is a fundamentally important cellular 81 regulatory process that is characteristically driven less effectively by partial relative to 82 full agonist ligands. Such agonist-selective endocytic discrimination depends on how 83 strongly drugs promote MOR binding to β-arrestins and this, in turn, depends on how 84 strongly they stimulate phosphorylation of the MOR cytoplasmic tail by GPCR kinases 85 (GRKs) from the GRK2/3 subfamily. While these relatively ‘downstream’ steps in the 86 agonist-selective endocytic pathway are now well defined, it remains unclear how 87 agonist-bound receptors are distinguished ‘upstream’ by GRKs. Focusing on GRK2 as a 88 prototype, we show that this single GRK subtype can distinguish the endocytic activities 89 of different MOR agonists in cells lacking other GRKs, and that agonist-selectivity is 90 introduced at the most upstream step of GRK2 binding to MOR. This interaction 91 requires prior membrane recruitment of GRK2 by its conserved PH domain and is 92 enhanced by phosphorylation of the MOR tail, but neither reaction can explain the high 93 degree of agonist-selectivity in the observed interaction of GRK2 with MOR. We identify 94 the N-terminal domain (NTD) of GRK2, which is identical in GRK3, as a discrete 95 element required for the full agonist-selectivity of MOR-GRK2 interaction and show that 96 the NTD is also required for GRK2 to promote MOR endocytosis after it is bound. We 97 propose a simple cellular mechanism of upstream agonist discrimination that is 98 organized as a series of biochemical checkpoints and utilizes the NTD as an agonist99 selective sensor.