Abstract Very low density lipoprotein (VLDL) has been isolated from normal (n) and dietary-induced hypercholesterolemic (hc) rabbits. Incorporation of the fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene into the lipid domains of both n VLDL and hc VLDL allowed assessment of the fluidity characteristics of these particles, utilizing fluorescence polarization techniques. Over the temperature range of 5° – 45°, the lipid region of n VLDL consists of an invariant phase, characterized by a microviscosity, η, at 30° of 0.6 ± 0.2 poise and a fusion activation energy, ΔE, of 7.6 ± 1.5 kcal/mole. The lipid region of hc VLDL, over the same temperature range, also is invariant and is characterized by a value of η at 30° of 4.6 ± 0.3 poise, and a ΔE of 7.8 ± 1.5 kcal/mole. Thus, large differences in the fluidit of the lipid in n VLDL and hc VLDL are evident, most probably due to the greatly increased content of cholesterol esters in hc VLDL, compared to n VLDL.