Dahl-Iwai salt-sensitive (S) and salt-resistant (R) rat strains were newly established as inbred strains. To characterize the strains, the Dahl-Iwai S and R rats were fed low-salt (0.3% NaCl) and high-salt (8.0% NaCl) diets from 5 weeks after birth, and systolic blood pressure and pathologic findings were examined at intervals. The distributions of alleles at 19 biochemical and immunologic loci also were examined in the aforementioned strains, together with those for the inbred SS/Sea and SR/Sea strains, which were derived from inbred SS/Jr and SR/Jr strains, respectively. The Dahl-Iwai S rats were hypertensive after 3 weeks of consuming the 8.0% NaCl diet and died from 6 to 10 weeks after the diet was initiated. Renal lesions developed after 4 weeks' consumption of the high-salt diet. The Dahl-Iwai S rats were not hypertensive until at least the age of 21 weeks while they consumed the 0.3% NaCl diet, whereas it was reported that the SS/Jr rats became hypertensive at about 20 weeks of age when they consumed the low-salt diet. The Dahl-Iwai R rats were normotensive whether fed the 0.3 or 8.0% NaCl diet. Hydronephrosis was not observed in the Dahl-Iwai R rats, though it develops in SR/Jr rats with high frequency. Different distributions were detected for kidney alkaline phosphatase-1 (Akp-1) and amylase-1 (Amy-1) alleles between the Dahl-Iwai S and SS/Sea strains, and for esterase-14 (Es-14) and seminal vesicle protein-1 (Svp-1) alleles between the Dahl-Iwai R and SR/Sea strains. The phenotypic differences between the substrains of inbred Dahl rats could be ascribed to different genetic backgrounds.