Microcystin-LR(MC-LR), a cyanobacterial toxin produced by Microcystis aeruginosa, causes severe hepatotoxicity. Here we investigated the morphologic changes of rat hepatocyte spheroid induced by exposure of MC-LR($10^{-6}M$) in vitro. In addition, to determine the effects of such toxin in the process of hepatocyte spheroid formation, primarily isolated hepatocytes were incubated with MC-LR and the process of spheroid formation was observed. In both hepatocyte spheroid and suspension culture systems, the morphologic changes caused by MC-LR were noticible at 5 min post exposure and were characterized by the loss of microvilli, cytoplasmic vacuolation, the accumulation of lipid droplets, and blob formation. Especially, the size and numbers of blob on the cell surface were increased as the incubation time prolonged and the appearance of electron dense bodies were observed in the cytoplasm of hepatocyte at 20 min post exposure. Furthermore, bile canaliculi-like structures in the hepatocyte spheroids were slightly widened and the process of spheroids formation was inhibited in the isolated hepatocytes incubated with MC-LR. These results indicate that morphologic changes in. the hepatocyte membrane and organelles seem to be typical events in showing the MC-LR induced hepatotoxic effects and the spheroid culture method might be a useful experimental tool to evaluate hepatoxicity since it reflects the in vivo status of hepatocytes.