Dietary phosphate restriction produces an adaptive increase in renal tubular Na/Pi cotransport. A similar adaptation occurs during phosphate depletion in opossum kidney cells, a continuous line of cultured renal epithelial cells. We investigated the cellular changes associated with adaptation to phosphate depletion in OK cells, in isolation from the complex systemic changes that occur with in vivo phosphate restriction. Phosphate depletion for up to 24 hours was associated with increases in Na/Pi cotransport activity, Na/Pi cotransporter mRNA, and Na/Pi cotransporter protein. Moreover, the increases in Na/Pi cotransport, and Na/Pi cotransporter mRNA and protein occurred at physiologically relevant degrees of phosphate restriction. The experimental results suggest that increases in Na/Pi cotransporter mRNA and protein may mediate the increase in Na/Pi cotransport activity in OK cells during phosphate depletion. © 1995 Academic Press, Inc.