Plasma triglyceride (TG) turnover measurements were carried out in 92 healthy normolipaemic and 30 hypertriglyceridaemic subjects. The analysis of the data revealed an age-dependence of the efficiency with which triglyceride particles were removed from the circulation, the elimination capacity being lower in older than in younger subjects. The population at large displayed heterogeneity with regard to maximal removal capacity, as already reported. Of the three groups into which the subjects were divided according to elimination capacity, those of lowest capacity were proportionately more numerous in the hypertriglyceridaemic than in the control series. This indicates that high synthesis rate and low elimination capacity are both involved in the development of hypertriglyceridaemia in man. The TG turnover test is thought to be useful in analyses concerning the heritability of mixed hyperlipidaemic states. A statistically significant decrease of plasma lipoprotein lipase with age was established using a matched series, thus confirming earlier observations and demonstrating that turnover tests and analyses of TG-removing enzymes are in good agreement.