In this study, we have investigated the effects of alimentary lipemia in 15 normotriglyceridemic individuals on high density lipoproteins2 (HDL2) with respect to structure, composition, and substrate efficacy for hepatic lipase in vitro. In the study subjects, HDL2 levels ranged widely from 4.7 to 151.7 mg/dl plasma. HDL2 were isolated in the postabsorptive (pa) state and in the postprandial (pp) state, i.e., 7 h after ingestion of a standard fatty meal. In going from the pa state to the pp state, HDL2 exhibited higher flotation rates and lower densities due to a decreased proportion of protein (38.7----36.2%) and a higher abundance in phospholipid (32.5----34.9%). There was a variable increase in triglyceride at the expense of cholesteryl esters; this increase was correlated positively with the magnitude of pp lipemia (r = 0.69, P less than 0.01) and inversely with HDL2 levels (r = -0.72, P less than 0.01). Hdl2 fractions were incubated with human hepatic lipase in vitro. Product lipoproteins formed from lipolysis of pa-HDL2 and triglyceride-poorer pp-HDL2 were reduced in phospholipid content (by 25 and 50%, respectively) but remained in the size and density range of native HDL2. By contrast, a major fraction of triglyceride-richer pp-HDL2 was converted to particles with density, size, and apoprotein composition of native HDL3. Changes consistent with these findings in vitro were observed in vivo also, where 15 h postprandially, individuals with high-level lipemia showed a decrease in HDL2 and rise in HDL3, while those with lower-level lipemia did not. This study indicates that the magnitude of postprandial lipemia determines the proportion of triglyceride in pp-HDL2, which in turn determines whether or not HDL2 are converted to HDL3 by hepatic lipase action.