Body fluid volumes and their relation to mean arterial pressure and plasma renin activity (PRA) were examined in heminephrectomized rats after 4 wk of treatment with deoxy-corticosterone acetate (DOCA) and placed on one of three levels of salt intake, either high (D-HS), normal (D-NS), or low (D-LS); sham-operated rats, which received heminephrectomy and no DOCA treatment, also received high (S-HS), normal (S-NS), or low (S-LS) intakes of salt. Body fluid volumes were measured as the distribution volumes of radioiodinated serum albumin, 35SO4, and tritiated water for plasma volume (PV), extracellular fluid volume (EFV), and total body water (TBW), respectively. Approximately the same degrees of hypertension occurred in the D-HS and D-NS rats, but the D-LS rats were normotensive. PV and EFV were increased only in the D-HS rats, with no prominent changes occurring in the D-NS rats. Intracellular fluid volume (ICF) was not changed in the D-NS rats when compared with the S-NS rats. The ratios of PV/EFV and EFV/TBW in the DOCA-treated groups on high or normal salt were not different from their controls. PRA was greatly suppressed in the D-HS and D-NS rats when compared with all other groups. In another group of D-HS rats, sodium was restricted for 2 wk; in this group the mean arterial pressure fell to control levels without significant changes in PV, but interstitial fluid volume was reduced to normal levels. These results demonstrated that 1) in DOCA-salt hypertensive rats there is expansion of body fluid volumes that are proportionally distributed among the PV, EFV, and ICF; 2) increases in body fluid volumes are not necessary for DOCA to maintain hypertension; 3) a certain minimal amount of dietary sodium is necessary for the development and maintenance of hypertension; and 4) following DOCA treatment the suppression of PRA is not due solely to expansion of body fluid volumes.