MC-LR belongs to a family of cyanobacterial toxins. MC-LR acts as serine-threonine phosphatase-1 and -2A inhibitor. Chronic intoxication with low doses of this toxin promotes liver tumour formation and induces kidney injury. The aim of the study was to evaluate whether chronic exposure to relatively low doses of MC-LR has toxic effects on hearts of treated animals. Male adult Wistar rats were treated every second day for 8 months with MC-LR (10microg/kg i. p., N = 5). Control groups were treated either with a vehicle (ethanol and methanol 4 : 1 v/v; N = 5) or with physiologic saline (N = 4). We found that MC-LR could induce enlargement of cardiomyocytes (MC-LR = 20.984microm+/-1.351, vehicle=17.454microm +/-0.518, saline = 15.996microm+/-1.430), loss of cell cross-striations, lower myofibril volume fraction (MC-LR = 0.3657mm(3)/mm(3) +/- 0.0337, vehicle=0.4716mm(3)/mm(3) +/-0.0086, saline = 0.4793 mm(3)/mm(3)+/-0.0101), fibrosis (MC-LR = 0.0747mm(3)/mm(3)+/-0.01288, vehicle = 0.0275 mm(3)/mm(3) +/- 0.0076, saline = 0.0309mm(3)/mm(3) +/-0.0074) and mononuclear infiltration in the interstitial tissue. The TUNEL staining of the heart sections of rats in all groups showed no apoptotic cells. We may conclude that long-term exposure to relatively low doses of MC-LR represents a considerable risk of injury of the heart.