Cell growth and synchronization, gene expression and regulation, maintenance of epithelial cell polarization are major functional aspects of the nephron which have been difficult to approach with conventional preparations. Certainly, without the introduction of tissue culture techniques, the opportunity to analyze these issues could not have been afforded. Several renal epithelial cell lines with differentiated characteristics of proximal and distal segments of the nephron are already available. LLC-PK1, derived from a normal Hampshire pig kidney, shows multiple differentiated characteristics of in vivo epithelia. Specifically, this cell line has Na+-dependent sugar, amino acid, and phosphate cotransport systems with similar characteristics as those present in the renal proximal tubule. The expression of the Na+-dependent sugar transport system in LLC-PK1 cells depends on the growing conditions of the cells. For instance, isolated cells obtained by trypsin-EDTA treatment of confluent monolayers or exponentially growing cells do not express the Na+-dependent sugar transport system. Full expression occurs after the monolayer reaches confluency. Expression of the transporter can also be modified by agents that affect the differentiation of other cellular systems, such as the Friend erythroleukemia cells. The expression of the Na-sugar cotransport system is also regulated by the concentration of glucose in the growth medium. Low glucose concentration increases the sugar influx through the Na+-coupled apical membrane transporter by increasing the number rather than the affinity of the transporter. This effect appears to be mediated through the sugar metabolism of the cell. The expression of the Na+-amino acid cotransport system in LLC-PK1 cells and the epithelial cell line MDCK derived from a normal dog kidney also responds to regulatory signals associated with cell growth or amino acid deprivation. LLC-PK1 cells and the cell line OK derived from an opposum kidney, shows a Na+-dependent phosphate cotransport system modulated by parathyroid hormone and cyclic nucleotides that will prove to be an excellent model not only to study the mechanisms of the action of the hormone, but to study the mechanisms involved in the expression and modulation of the Na+-dependent phosphate cotransport system.