When injected intravenously in rats, emulsion models of triacylglycerol-rich lipoproteins were metabolized like natural lipoproteins and during the hydrolysis of emulsion triacylglycerols, a large fraction of the emulsion phosphatidylcholine was transferred to the plasma high-density lipoproteins. The removal from plasma of emulsion phosphatidylcholine was followed for 2 h in unanaesthetized rats. The half-lives for removal of phospholipid after injection of emulsions stabilized with dioleoylphosphatidylcholine or 1-palmitoyl-2-oleolyphosphatidylcholine were 58-63 min when traced with isologous label. In comparison, the published half-lives of HDL mixed phospholipids in rats are approx. 40 min, indicating that much of the clearance of the emulsion phospholipid could be accounted for by HDL catabolism. Measured LCAT activity was sufficient to account for not more than 2% of the catabolism of the HDL phospholipids labelled by this physiological procedure. Removal from plasma of label was more rapid when the same emulsions were labelled with tracer amounts of the heterologous dipalmitoylphosphatidylcholine, showing that individual phosphatidylcholine species were handled distinctly even when present only in tracer amounts in a bulk of another phosphatidylcholine differing in acyl chains.