Several abnormalities of cation transport have been described in the Milan hypertensive rats (MHS). In this study we examined Na,K-ATPase activity in proximal convoluted tubules (PCT) cells and medullary thick ascending limb of Henle cells (TAL) from MHS and from the Milan normotensive rats (MNS). Na,K-ATPase activity was determined as 32P-ATP hydrolysis in single tubule segments. Na,K-ATPase activity (pmol Pi/mm t/h) was significantly higher in MHS than MNS both in PCT (903 +/- 227 n = 8 v 506 +/- 285 n = 12) and TAL (4324 +/- 800 n = 5 v 3063 +/- 625 n = 5). Na,K-ATPase dependent respiration was determined in PCT cell from MNS and MHS. Under basal condition Na,K-ATPase dependent respiration (mumol O2/mg protein/h) was higher in MHS than in MNS (24.2 +/- 1.8 n = 5 v 16.1 +/- 0.4 n = 5). When the cells were Na loaded by amphotericin Na,K-ATPase dependent respiration increased significantly more in MHS than MNS (38.4 +/- 1.6 v 26.8 +/- 2.2 n = 4). Thus, Na,K-ATPase activity is higher in renal tubule cells both at normal intracellular Na and after the cells have been Na loaded. The results indicate that regulation of Na homeostasis in renal tubule cell is different in MHS and MNS.