Activation of the G protein-coupled TRH receptor leads to its phosphorylation and internalization. These studies addressed the fundamental question of whether phosphorylation regulates receptor trafficking, or endosomal localization regulates the phosphorylation state of the receptor. Trafficking of phosphorylated and dephosphorylated TRH receptors was characterized using phosphosite-specific antibody after labeling surface receptors with anti-HA antibody. Rab5 and phospho-receptor did not co-localize at the plasma membrane immediately after TRH addition but overlapped extensively by 15 min. Dominant-negative Rab5-S34N inhibited receptor internalization. Later, phospho-receptor was in endosomes containing Rab5 and Rab4. Dephosphorylated receptor co-localized with Rab4 but not with Rab5. Dominant-negative Rab4, -5, or -11 did not affect receptor phosphorylation or dephosphorylation, showing that phosphorylation determines localization in Rab4⁺/Rab5^- vesicles and not vice versa. No receptor co-localized with Rab7; a small amount of phospho-receptor co-localized with Rab11. To characterize recycling, surface receptors were tagged with antibody, or surface receptors containing an N-terminal biotin ligase acceptor sequence were labeled with biotin. Most recycling receptors did not return to the plasma membrane for over two hours after TRH was removed, whereas the total cell surface receptor density was largely restored in under one hour, indicating that recruited receptors contribute heavily to early repopulation of the plasma membrane.