Proximal (LLC-PK1) and distal (MDCK) renal epithelial cell cultures were used to investigate early biochemical changes in cellular metabolism following exposure to cyclosporine A (CsA).3H-thymidine and3H-leucine incorporation into the cells were used as indices of DNA and protein synthesis. The cells were exposed to concentrations of CsA ranging from 0.2 μg/ml to 20 μg/ml. By 20 hr there was a decrease in the total cell count at concentrations of 10 and 20 μg/ml that was more pronounced by 5 days of exposure. At 5 days there was also a reduction in cell count at the lower concentrations of CsA. There was an initial increase in DNA and protein synthesis at 2 hr with inhibition of DNA synthesis evident by 20 hr. Protein synthesis was increased in the LLC-PK1cells and decreased in the MDCK cells. At 5 days there was evidence of increased DNA and protein synthesis, most marked in the remaining viable cells exposed to the higher concentrations of CsA. Similar alterations in cellular metabolism were evident when the cells were exposed to the immunologically inert cyclosporine H (D-N-MeVal11-Cs). These studies demonstrate that cyclosporine produces alterations in cellular function as early as 2 hr after exposure to the drug. At the lower concentrations there is evidence of sublethal cellular toxicity and cellular regeneration. The toxicity appears to be related to the molecular structure of cyclosporine and its incorporation into cell membrances. We postulate that cyclosporine nephrotoxiciy is the summation of several subtoxic alterations in cellular function the final expression of which is modified by other factors affecting renal function.