Cyclosporine (CSA) nephrotoxicity was investigated in the rat. Morphologically, CSA nephrotoxicity is characterized by the following features: (a) Tubular inclusion bodies (TIB) corresponding to autolysosomes and giant mitochondria; (b) tubular vacuolization (TV) due to dilatation of the endoplasmatic reticulum; (c) tubular microcalcification (TM); and (d) tubular regeneration (TR). The morphologic features are limited to (a and b) or predominate (c and d) in the proximal tubule. CSA tubulopathy as defined above is dose dependent, independent of the route of a drug administration, develops quickly (within a week), is reversible after CSA withdrawal, is more pronounced in male than in female rats and shows no clear cut differences in various rat strains. Body weight shows a dose dependent reduction. The functional changes are usually slight. No significant correlation exists between functional changes and morphologic lesions. The type of CSA nephrotoxicity in the rat is very similar to CSA-tubulopathy in man but associated with less severe functional changes.