The time course of the effects of dietary cholesterol on the lipid composition of liver, plasma and red cells of guinea pigs, of the pathological changes of tissues and of hematological parameters was studied. The purpose was to identify the primary injury and so to contribute to the eludication of the mechanism(s) for the development of the hemolytic anemia observed in this species after long-term cholesterol supplementation. The results showed that the initial effects observed within 1 week occur in the liver lipids and histology accompanied by changes in plasma and RBC lipids. These events were followed by further, slower increases of tissue lipids without major qualitative changes. The earliest signs of an anemia were observed between weeks 5 and 7. We conclude that the primary insult of cholesterol is liver damage leading to the production of abnormal plasma lipoproteins which in turn cause a net increase of RBC-cholesterol that is accompanied by their morphological abnormalities. The hemolytic anemia does not seem to be caused directly by either the altered composition of RBC nor their altered morphology.