The polyglutamine tract, beginning at codon 75 in the N-terminal modulatory domain of rat glucocorticoid receptor (rGR), was analyzed in 61 inbred strains and 155 wild caught Rattus norvegicns. A discontinuous distribution of repeat lengths was found (7, 17–23 repeats). To investigate the possible significance of this distribution, full-length rGR cDNAs with 7, 18, 20, and 21 CAG repeats were expressed in CV-1 cells, and the resulting GR protein analyzed by Western blots and extensive Scatchard analyses. The quantity and steroid binding capacity of GR, together with the binding affinities for dexamethasone and corticosterone, were found to be indistinguishable for the four repeat alleles. From the sequencing of four inbred strains CAG repeat variants were found to be flanked by silent allelic substitutions at nucleotide positions 198, 531, and 711. The four variable sites extended over 471–519 bp of coding sequence, forming six Grl haplotypes. The results are discussed in the light of genetic studies on the Milan hypertensive and normotensive strains of rat. Codon sequence of rat GR required amendment at the following residues: D98, G226, D260, P600, and F602.