Previous investigations have reported differences in the affinity of hepatic glucocorticoid receptors (GR) for glucocorticoids between the Milan hypertensive (MHS) and normotensive (MNS) strains (Kenyon et al., 1994), the Zucker lean and obese strains (White and Martin, 1990) and hypertensive SHR and normotensive WKY strains of rat (Panarelli et al., 1995). To determine the possible causes of this phenotypic variation, a series of investigations were carried out into the molecular genetics of strain specific allelic variants of rat GR.