1. The metabolism of the major proteins of plasma high-density lipoprotein (HDL), apoproteins AI and AII, have been studied in 10 normotriglyceridaemic subjects and in 11 hypertriglyceridaemic subjects (plasma triglyceride 4.5--25 mmol/l) by kinetic analysis of the plasma specific radioactivity versus time curves of the apoproteins after intravenous injection of autologous 125I-labelled high-density lipoprotein. 2. The specific radioactivity versus time curves of both apoproteins (followed for 14 days) were bi-exponential in all subjects. 3. The plasma apoprotein AI and AII concentrations were significantly lower in the hypertriglyceridaemic subjects than in the normotriglyceridaemic subjects. Kinetic analysis showed that this was associated with a lower rate of synthesis of apoprotein AI (P< 0.01) and a higher fractional catabolic rate of apoprotein AII (P< 0.01) in the hypertriglyceridaemic group. 4. There were no significant differences between the two groups in the synthetic rate of apoprotein AII, the fractional catabolic rate of apoprotein AI or the intravascular/extravascular distributions of the apoproteins. 5. Thus hypertriglyceridaemia appears to be frequently associated with divergent abnormalities of the metabolism of the major high density lipoprotein apoproteins.