The aim of this work was to compare the disappearance rate of human and rat intermediate density lipoproteins (IDL) using the rat liver perfusion system. Human and rat IDL were produced in vitro by incubating human or rat very low density lipoproteins (VLDL) with either rat post-heparin plasma (method I) or a resolubilized isopropanol precipitate of rat post-heparin plasma (method II). With both methods, the degree of triacylglycerol lipolysis was approximately 55%. The different preparations of IDL were labelled with 125I and added to perfusates of rat livers. The disappearance rates of 125I-labelled IDL were monitored by measuring the radioactivity associated with apolipoprotein (apo) B in the perfusate during a 15-min period. Both human and rat IDL prepared with method I had an increased apoE to apoC ratio as compared with their native counterparts. Furthermore, human IDL had a significantly higher apoE to apoC ratio than rat IDL. However, when IDL were produced in the absence of exchangeable apolipoproteins (method II), no change in the apoE to apoC ratios was observed for the transformation of VLDL to IDL and the ratios were similar for human and rat IDL. Despite these differences, human IDL were always removed at a lower rate than rat IDL. The only striking difference between the two types of IDL made by method II was that the apoB100 to apoB48 ratio was considerably higher in human than in rat IDL. These results suggest that the apoB100 to apoB48 ratio is likely to be responsible for the observed differences in liver uptake between rat and human IDL.