Plasma squalene concentration is increased in hypertriglyceridemia. In 24 normotriglyceridemic and 12 hypertriglyceridemic subjects, whole plasma squalene correlated strongly with plasma triglyceride (r = 0.973, P < 0.001) in the latter. In normal postabsorptive plasma, squalene was found in each lipoprotein fraction, 50.8% in very low density lipoprotein, 25.6% in low density lipoprotein, and 23.6% in high density lipoprotein. When plasma triglyceride was increased by dietary intake in humans or by experimental diabetes in rats, plasma squalene increased correspondingly. Conversion of [(14)C]mevalonic acid into [(14)C]squalene and kinetic analysis of [(14)C]squalene die-away curves were studied in 17 subjects. Hypertriglyceridemia significantly increased the estimated metabolically active plasma squalene pool. This together with an increase in radioactivity of squalene (dpm/ml plasma) in hypertriglyceridemia suggested that squalene production was increased. Squalene specific activity curves in lipoprotein fractions from four chylomicronemic subjects demonstrated that each fraction had newly synthesized squalene and that total plasma squalene kinetics represent the composite of several individual die-away curves. We conclude that squalene in whole plasma and in lipoprotein fractions varies directly with triglyceride content. Hypertriglyceridemia expands the plasma pool of metabolically active squalene, and each lipoprotein fraction contains squalene that is metabolically active in cholesterol synthesis.