1. Exogenous sex steroids appear to influence lipoproteins in a manner that is a caricature of the effects of endogenous sex steroids: Estrogens raise HDL (selectively HDL2) and lower LDL; Androgens lower HDL (selectively HDL2), while raising LDL. 2. Exogenous sex steroids are likely to affect LDL metabolism via effects on the LDL receptor; Estrogens increase LDL receptor activity (in non-human species at both the hepatic cellular and mRNA levels, though this is yet to be confirmed in humans); ??Androgens decrease LDL receptor activity (yet to be tested in either human or non-human species). 3. Exogenous sex steroids appear to alter HDL levels predominantly via modulation of HDL catabolism; Estrogens retard HDL catabolism (33) (and may also increase apo A-I synthesis and HDL production); Androgens accelerate HDL catabolism (30). 4. Modulation of HDL (and possibly LDL) metabolism by sex steroids may be mediated by alterations in hepatic triglyceride lipase (HTGL) activity.