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Microcystins damage liver tissue by disorganizing the cytoskeleton and inhibiting serine/threonine protein phosphatases. Liver cells shrink and blood penetrates the liver tissue. Internal bleeding may lead to haemorrhagic shock. Water pollution associated with the blooming of toxic cianobacteria as a result of intensive agriculture has been well-document­ed in the north-eastern Slovenia. We undertook to develop a method for isolation of hepatocytes, and to assess the potency of microcystin RR isolated from the polluted water in Slovenia. By acute exposure of rats to a mixture of microcystins RR and LR we wanted to ascertain whether internal bleeding reflects changes in the haematocrit count. Morphological changes fol­lowing chronic exposure of animals to micro­cystin mixture were studied. Simultaneously, we tested the hypothesis that magnetic reso­nance (MR) imaging is a suitable technique for assessing acute and chronic toxicity of micro­cystins. The experiments were performed on 37 male Wistar rats. Twenty animals were sac­rificed during acute experiments. The effects of the chosen toxin on the isolated hepatocytes obtained by in situ perfusion technique were test­ed in half the animals, while in another half the toxin mixture effects were assessed in vivo using MR imaging. For chronic tests the animals were fed with microcystins for two months. Blood was drawn from the inferior caval vein for com­plete blood count and serum analysis. The liver was removed and weighed and liver specimens were obtained for histological examination. The hepatocyte isolation method afforded a 83% cell survival. The hepatotoxicity of microcystin RR was comparable to that of microcystins used in previous studies. In acute experiments, the weight of the liver increased by 35% and its cross-sectional area by 19.3% on MR images. Histological studies revealed focal intrahepatic haemorrhages. The haematocrit decreased by 13%. Chronic administration of microcystins led to unspecific changes in the morphological pattern and MR images. Liver function tests showed several abnormalities, including a drop in free serum iron by 39.5%. The mean cor­puscular haemoglobin, mean corpuscular haemoglobin levels and mean corpuscular vol­ume decreased. MR imaging is a suitable method for assessing acute intoxication. Internal haemorrhage can be monitored by haematocrit determinations. Pathological liver tests and microcytic hypochromic anaemia constitute changes characteristic of chronic intoxication, whereas histological studies and MR imaging yield only unspecific findings.