The three vastatins examined, lovastatin, simvastatin and pravastatin, are equally strong inhibitors of the sterol synthesis in human hepatocytes in culture with IC50-values of 4.1, 8.0 and 2.0 nM, respectively. However, in the human extrahepatic cells: umbilical vascular endothelial cells, retinal pigment epithelial cells, cornea fibroblasts and granulosa cells, pravastatin was much less inhibiting the sterol synthesis than lovastatin or simvastatin. It was observed as well that longer incubation with the vastatins resulted in higher IC50-values. In order to show that the feedback regulation mechanism for 3-hydroxy-3-methylglutaryl-coenzyme A reductase was involved in this phenomena mRNA levels were measured in human vascular endothelial cells after incubation with the vastatins for 3.5 h and for 20 h. Indeed, lovastatin and simvastatin gave rise to higher levels of HMG-CoA reductase mRNA after 20 h than after 3.5 h of incubation. The differences observed in different human cell types can be explained by supposing that pravastatin is transported into the human hepatocyte via a liver-specific transporter. This was supported by the results of uptake experiments with 14C-labelled pravastatin and 14C-labelled simvastatin into human hepatocytes compared to that into human umbilical endothelial cells (as an example of an extrahepatic cell type). [14C]-Simvastatin was associated with both cell types, whereas [14C]-pravastatin was hardly associated with human endothelial cells, but to a similar extent as [14C]-simvastatin with human hepatocytes.