Intravenous injection of praseodymium nitrate into female Wistar rats results in liver damage. The aim of this study is to investigate the quality of serum high density lipoprotein content as an index for the severity and time course of liver damage and regeneration following the administration of praseodymium. Serum high density lipoprotein content drastically decreases to a minimum after 24 - 48 h, returning to control values after four days. Liver degeneration is characterized by some intracellular parameters, i.e. the nuclear RNA polymerase reactions, the ribosomal protein synthesis, hepatic spermidine concentration and the activities of serum transaminases (GOT, GPT) and the sorbitdehydrogenase. From the data it is evident that the time course of serum high density lipoprotein content follows the intracellular changes closely. Liver regeneration is represented by the ornithin decarboxylase, the deoxycytidylate deaminase, the thymidine kinase activities and the hepatic putrescine content. The time course of these parameters shows that the regeneration reaches a maximum after 3 - 4 days. In the serum, high density lipoprotein content reflects this process by returning to control values. From our data we conclude that serum high density lipoprotein content after i.v. administration of praseodymium can be considered as an expression of the functional state of the liver.