Mutations of the leptin receptor have been found to cause obesity in rodents. The fa mutation that is responsible for obesity in Zucker rats is a missense mutation (269 gln→pro) in the extracellular domain of the leptin receptor. We have characterized the effects of this mutation on the two major isoforms of the leptin receptor, Ob-Rb and Ob-Ra, by studying cell-surface expression, leptin binding affinity, signaling capacity, and receptor-mediated internalization and degradation of leptin in transfected mammalian cell lines. Both Ob-Rb^{269 gln→pro} and Ob-Ra^{269 gln→pro} have decreased cell-surface expression and decreased leptin binding affinity. Ob-Rb^{269 gln→pro} was shown to have defective signaling to the JAK-STAT pathway and markedly diminished ability to activate transcription of the egr-1 promoter. Constitutive ligand-independent activation of Ob-Rb^{269 gln→pro} was observed for activation of egr-1-luc but only under conditions when JAK2 was coexpressed with Ob-Rb^{269 gln→pro}. Finally, Ob-Ra^{269 gln→pro} has an increased ability to internalize leptin but is less efficient at degrading leptin, as compared with Ob-Ra. In conclusion, both Ob-Ra^{269 gln→pro} and Ob-Rb^{269 gln→pro} have multiple functional defects.