Intravenous fat tolerance was tested in two groups of patients given a continuous i.v. infusion of heparin for several days. One group of 11 patients with deep vein thrombosis (DVT) of the leg was given 25,000-35,000 IU heparin daily for 4-5 days. The other group comprised 10 patients who had central venous catheters (CVC) for total parenteral nutrition. These patients were given 20,000 IU heparin daily for 6 days as prophylaxis against CVC-related thrombosis. In the DVT group heparinization was associated with a 44% decrease in plasma fat removal capacity (P less than 0.05). This reduction persisted for 2 days after the discontinuation of heparin therapy. In the CVC group the plasma fat removal capacity decreased by 29% during heparinization (P greater than 0.05, NS). During heparinization activated partial thromboplastin time was more than three times the basal value in the DVT group but less than twice those in the CVC group. One week after the heparin therapy the serum triglyceride levels were higher in both groups compared with initial values (DVT group: 1.2 +/- 0.2 s.e. mean vs. 1.7 +/- 0.3 mmol/l; P less than 0.05. CVC group: 1.0 +/- 0.1 vs. 1.4 +/- 0.2 mmol/l; NS). The possibility that full-dose heparinization reduces plasma fat removal capacity and that this may be due to a partial depletion of lipoprotein lipase stores is discussed.