The effect of cyclosporine A (CsA) on lipid peroxidation (LPO) was assessed in renal cortical slices and renal microsomes. Cortical slices were incubated with 1500μg/ml CsA and microsomes with 0.5–20μg/ml under identical conditions (pH 7.4,37d`C) for 3 hours, and LPO monitored by the formation of malondialdehyde (MDA). CsA at concentrations of 3μg/ml and higher caused a significant increase MDA in microsomes and renal cortical slices showed a time dependent release of MDA into the incubation medium. The influence of CsA on tetraethammonium (TEA) and p-ami-nohippurate (PAH) accumulation in renal cortical slices was investigated for up to 3 hours with concentration of CsA from 10 to 1000μg/ml. CsA caused a time-and concentration-dependent decrease of TEA accumulation and higher concentrations of CsA decreased PAH accumulation in renal cortical slices. The results add further evidence to the suggestion that lipid peroxidation participate in CsA-induced impairment of kidney function.