—To investigate the metabolism of nascent HDLs, apoA1/phosphatidylcholine (apoA1/PC) discs were infused IV over 4 hours into 7 healthy men. Plasma total apoA1 and phospholipid (PL) concentrations increased during the infusions. The rise in plasma apoA1 was greatest in small preβ-migrating particles not present in the infusate. Total HDL unesterified cholesterol (UC) also increased simultaneously. After stopping the infusion, the concentrations of apoA1, PL, HDL UC, and small preβ HDLs decreased, whereas those of HDL cholesteryl ester (CE) and large α-migrating apoA1 containing HDLs increased. ApoB-containing lipoproteins became enriched in CEs. Addition of apoA1/PC discs to whole blood at 37°C in vitro also generated small preβ HDLs, but did not augment the transfer of UC from erythrocytes to plasma. We conclude that the disc infusions increased the intravascular production of small preβ HDLs in vivo, and that this was associated with an increase in the efflux and esterification of UC derived from fixed tissues. The extent to which the increase in tissue cholesterol efflux was dependent on that in preβ HDL production could not be determined. Infusion of discs also reduced the plasma apoB and apoA2 concentrations, and increased plasma triglycerides and apoC3. Thus, nascent HDL secretion may have a significant impact on preβ HDL production, reverse cholesterol transport and lipoprotein metabolism in humans.