Renal brush-border membrane vesicles isolated from rats kept for 6-8 weeks on a low-phosphate diet (0.15% of dry matter) showed a markedly faster Na(+)-dependent phosphate uptake than did membrane vesicles isolated from animals kept on a high-phosphate diet (2% of dry matter). Phosphate-uptake rate by brush-border membrane vesicles isolated from animals on a low-phosphate diet remained significantly increased after acute parathyroidectomy. Dietary adaptation was also observed in animals that had been parathyroidectomized before exposure to the different diets. In animals on the low-phosphate diet parathyrin administration inhibited phosphate uptake by brush-border vesicles only if the animals were repleted with P(i) (5ml of 20mm-NaH(2)PO(4)) 1h before being killed. After acute phosphate loading and parathyrin administration the difference in the transport rate between the two dietary groups remained statistically significant. The results suggest that the adaptation of proximal-tubule phosphate transport to dietary intake of phosphate is reflected in the Na(+)/phosphate co-transport system located in the luminal membrane of the proximal-tubule cell. Since the dietary effects on phosphate transport by brush-border membranes are only partially reversed by acute changes in parathyrin concentration and are also observed in chronically parathyroidectomized animals, the adaptation of the Na(+)/phosphate co-transport system to dietary phosphate intake seems to involve an additional mechanism independent of parathyrin.