In 26 male patients with primary hypertriglyceridemia (HTG) the radioglycerol labelling technique was used to assess triglyceride production rates and fractional catabolic rates pertaining to the rapidly turning-over compartment of endogenously produced triglycerides (TG). On the basis of these data the patients were allotted to 3 groups: group I with predominating TG overproduction, group II with predominating disturbances of TG removal from the bloodstream, and group III with a combination of both dysfunctions. All patients were examined prior to and after 4 weeks of drug intake at the metabolic ward. Each group got a medical treatment aiming at removing the pathogenetic cause of the HTG. Biguanides used in group I decreased TG production rates without affecting fractional catabolic rates. In group II an anabolic steroid (chlormethyltestosterone) proved to be able to improve TG removal, but this improvement was accompanied by a significant lowering of HDL cholesterol and an increase in LDL cholesterol. In contrast, clofibric acid influenced both overproduction and removal impairment in group III. Our results give an insight into mechanisms of action of certain lipid-lowering drugs providing a basis for future improvements in the elimination of the risk factor HTG.