During the past few years, new methods have evolved that allow much more accurate measurements in vivo of the rate constants for the physiological processes that determine sterol balance in the whole animal and that dic-tate the level of circulating cholesterol carried in low density lipoproteins (LDL-C). The purpose of this review, therefore, is I) to summarize the quantitative physiology of the control of cholesterol balance and LDL-C concentrations in all species in which such data are now available; 2) to point out where commonly held concepts have been proved incorrect by these new measurements; and 3) to emphasize those specific areas where further experimental data are required to better understand these regulatory processes.