The concentration of cholesterol in high density lipoproteins (HDL) has been showed to be dramatically decreased during pregnancy in Macaca nemestrina. HDL were isolated from females of this species at various stages of pregnancy to determine if pregnancy also alters their composition and size. The chemical compositions of the HDL were determined nad found different in pregnant animals; the mass ratio of surface (coat) to center (core) constituents was higher, suggesting that the average size of HDL decreased during pregnancy. When measured chromatographically, the average size of HDL was found to decrease during pregnancy. This change in HDL size was accompanied by a minor alteration in apolipoprotein distribution.