Blood ciclosporin (Cs) metabolite pattern in 58 liver grafted patients was routinely monitored by HPLC from the first Cs dose after transplantation until discharge from hospital. Eighteen patients with normal kidney function were allocated to Group I and 14 patients in Group II suffered Cs nephrotoxicity during their clinical course. There were no significant differences between both groups in blood Cs level, kidney function before transplantation, liver function or co-administration of other potentially nephrotoxic drugs. A correlation matrix involving both groups showed a significant correlation between the blood concentration of metabolite M1c9 and serum creatinine and urea, and an inverse correlation with creatinine clearance. During a nephrotoxic episode the blood concentrations of metabolites M1c9 and M1A were significantly elevated in patients in Group II. Analysis of the time course revealed significantly higher blood levels of M19 and M1c9 in Group II patients compared with those in Group I for the first 10 days after transplantation. Serum creatinine and urea concentrations remained significantly elevated, the creatinine clearance being significantly reduced throughout the period of observation. The elevated blood concentrations of ciclosporin metabolites M1c9 and M19 during nephrotoxic episodes suggest that these metabolites are associated with ciclosporin nephrotoxicity. It could not be decided if the elevated metabolite concentrations were the result of and/or the reason for impaired kidney function.