We previously reported that cholesteryl ester transfer protein (CETP) inhibitory peptides (designated $P_{28}$ and $P_{10})$ have anti-atherogenic effects in hypercholesterolemic rabbits (Biochim. Biophys. Acta (1998) 1391, 133-144). To further investigate those effects, we studied rabbit plasma that was collected after 30 h of a $P_{28}$ or $P_{10}$ injection. We found that there is a strong correlation between the in vivo CETP inhibition effects and alterations of lipoprotein particle size distribution in rabbit plasma, as determined on an agarose gel electrophoresis and gel filtration column chromatography. In vivo effects of the peptide were observed again in C57BL/6 mice that expressed simian CETP. The $P_{28}$ or $P_{10}$ peptide ($7;{mu}g/g$ of body weight) that was dissolved in saline was injected subcutaneously into the mice. The $P_{28}$ injection caused the partial inhibition of plasma CETP activity up to 50%, decreasing the total plasma cholesterol concentration by 30%, and increasing the ratio of HD/total-cholesterol concentration by 150% in the CETP-transgenic (tg) mice. The CETP inhibition by the $P_{28}$ or $P_{10}$ made alterations that modulated the size re-distribution of the lipoproteins in the blood stream. Particle size of the very low (VLDL) and low density lipoproteins (LDL) from the peptide-injected group was highly decreased compared to the saline-injected group (determined on the gel filtration column chromatography). In contrast, The HDL particle size of the $P_{28}$-injected group increased compared to the control group (saline-injected). The expression level of the CETP mRNA of the $P_{28}$-injected CETP-tg mouse appeared lower than the saline-injected CETP-tg mouse. These results suggest that the injection of the CETP inhibitory peptide could affect the CETP expression level in the liver by influencing lipoprotein metabolism.