Efficacy of 1-(theophyllin-7-yl)-ethyl-2-[2-(p-chlorophenoxy)-2-methylpropionate] (etofylline clofibrate, ML 1024, Duolip) and its molecule components (metabolites) of structurally similar theophylline esters and of clofibrate as standard was investigated in the artificial hyperlipidaemia of the rat. In accordance with former results etofylline clofibrate was antilipaemically active and, in contrast to similar esters and clofibrate, significantly decreased the cholesterol level. An investigation of the efficacy of its metabolites, either alone or in equivalent mixture, as well as of the standard clofibrate under fat diet demonstrated low efficacy of etofylline, but an increased activity in combination with clofibrate or clofibric acid. The activity of the combination is significantly superior to that of clofibrate under fat diet, but not under normal diet. The increased efficacy of etofylline clofibrate is undoubtedly an unusual potentiation, an additive effect of the metabolites can be excluded. Cholesterol and triglycerides are relevant parameters for the experimental evaluation of the efficacy. Measurement of total lipids offers no additional information. Substitution of triglycerides by total-beta-lipoproteins as parameter seems methodically useful, since values of cholesterol, triglycerides and beta-lipoproteins correlate well under normal diet.