Sodium-dependent transport of phosphate was studied in LLC-PK1 cells that had been deprived of phosphate (Pi). Compared with control cells (fed with 2 mM Pi) a twofold increase in the rate of Na-Pi cotransport was observed in cells incubated for 15 h in a phosphate-free medium, whereas transport of L-alanine and the specific activity of alkaline phosphatase were not changed. The same adaptive response was observed with apical membrane vesicles isolated from Pi-deprived cells. In both experimental systems Pi deprivation caused a change in the Vmax but not in the apparent Km (for Pi) of the cotransport system. Adaptation of the Na-Pi cotransport was triggered by free phosphate concentrations between 0 and 100 microM. Over the first 20 h the adaptive response was found to be a linear process that could be prevented by 70 microM cycloheximide. Adapted cells that were re-treated with phosphate showed a rapid (less than 3 h) decrease in the Na-Pi transport. The data suggest that LLC-PK1 cells adapt to low Pi conditions by increasing the rate of the Na-Pi cotransport, which is located in the apical membrane. Two mechanisms may be involved in the adaptive response: a long-term process involving new protein synthesis, and a short-term response involving activation-inactivation of transport systems already existing.