Stably transfected human kidney 293 cells expressing the wild type rat LH/CG receptor (rLHR) or receptors with C-terminal tails truncated at residues 653, 631, or 628 (designated rLHR-t653, rLHR-t631, and rLHR-t628) were used to probe the importance of this region on the regulation of hormonal responsiveness. The chosen cells line express comparable densities of cell surface rLHR, bind human CG (hCG) with high affinity, and respond to hCG with increases in cAMP and inositol phosphate accumulation. Cells expressing rLHR-wt or rLHR-t653 responded to hCG, or phorbol 12-myristate-13-acetate (PMA) stimulation with a similar increase in rLHR phosphorylation. Neither of these two stimuli increased rLHR phosphorylation in cells expressing rLHR-t631 or rLHR-t628, however. The cell line expressing rLHR-t653, the phosphorylation-positive receptor mutant, desensitized normally in response to PMA or hCG stimulation. This truncated form of rLHR also was down-regulated normally in response to hCG stimulation. In contrast, the cell lines expressing rLHR-t631 or rLHR-t628, the two phosphorylation-negative receptor mutants, showed a delay in the early phase of hCG-induced desensitization, a complete loss of PMA-induced desensitization, and an increase in the rate of hCG-induced receptor down-regulation. These results clearly show that residues 632-653 in the C-terminal tail of the rLHR are involved in PMA-induced desensitization, hCG-induced de-sensitization, and hCG-induced down-regulation. These results also establish a positive correlation between rLHR phosphorylation and desensitization and a negative correlation between rLHR phosphorylation and down-regulation.