The microcystins (MCs) LR and YR are hepatotoxins produced by some species of freshwater cyanobacteria. Only a few reports on the acute nephrotoxicity of MCs have been published to date. Here we investigate the effects on rat kidneys of chronic administration of relatively low doses of MC LR and MC YR. Male Wistar rats were injected every second day for 8 months with MC LR (10 microg/kg i.p., n=5) and MC YR (10 microg/kg i.p., n=5). The control group was treated with vehicle, a mixture containing 0.8 % ethanol and 0.2 % methanol in 0.9 % saline (3.7 ml/kg i.p., n=5). We found that MCs could induce damage to the kidney cortex and medulla. The lesions mainly consisted of damaged and dilated tubuli filled with homogenous eosinophil material. We conclude that long-term exposure to relatively low doses of MCs poses a considerable risk for kidney injury.