In the present study, very low density lipoprotein (VLDL, d less than 1.006) in cardiac lymph was characterized to determine its role as a metabolic substrate in the interstitial compartment. A major efferent cardiac lymph trunk was cannulated in fasting (18 h) dogs (20-27 kg). Three to five millilitres of lymph were collected over 3-4 h at 4 degrees C. Cardiac lymph VLDL concentration was 1.7 +/- 0.7 mg protein . 100 mL-1 compared with 1.8 +/- 0.8 mg protein . 100 mL-1 in plasma. The VLDL triglyceride concentration in lymph was 1.0 +/- 0.3 mg triglyceride . 100 mL-1 with triglyceride/protein ratio of 0.9 compared with plasma VLDL triglyceride of 5.0 +/- 1.6 mg . 100 mL-1 with a triglyceride/protein ratio of 5.5. Electron microscopy of VLDL revealed globular particles with a mean diameter of 388 A in lymph and 661 A in plasma. Thus, cardiac lymph VLDL are smaller and contain less triglyceride per particle than plasma VLDL. Following i.v. administration of human 125I-labelled low density lipoprotein ([125I]LDL, d 1.025-1.045), cardiac lymph/plasma LDL specific activity ratio was 0.52 +/- 0.15 (n = 3) and 0.55 +/- 0.15 (n = 4)) at 3 and 27 h, respectively. The fact that the specific activity ratio did not reach 1 at plateau suggests continuous addition of unlabelled LDL in the cardiac interstitium, presumably from VLDL precursors. These findings demonstrate that on a protein basis the concentration of VLDL in cardiac lymph equals that of plasma, and also suggests that VLDL degradation and LDL production occur in the cardiac interstitial space.