A method was developed for chronic catheterization of the left thoracic lymph duct at the base of the neck in the sheep fetus, which did not disrupt the other major lymphatic vessels that join the venous circulation at the same location. The lymphatic catheter was connected to a catheter in a jugular vein when lymph flow was not being recorded, so that the lymph continuously returned to the fetal circulation. Special consideration of catheter size to minimize flow resistance and treatment to prevent clotting were required. Individual animals were maintained up to 17 days with lymph flow continuing. In 13 fetuses averaging 128 days gestation (term = 147 days) at the time of catheterization, lymph flow rate was measured for 1 h each day for the first 7 postsurgical days with an on-line computer technique that continuously calculated lymph flow rate. Lymph flow averaged 0.64 +/- 0.17 (SD) ml/min in fetuses weighing 2.3-4 kg and tended to undergo a nonsignificant increase with time. Lymph and plasma protein concentrations did not change with time. In individual fetuses, large spontaneous variations in lymph flow rate occurred over periods of several seconds to a few minutes. Analysis showed that these variations in flow rate were not associated with fetal breathing movements. Thus the present study describes a technique for studying the dynamics of lymph flow in the unanesthetized sheep fetus in utero over a time period limited primarily by the length of gestation. In addition, it appears that thoracic duct lymph flow rate in the fetus per unit body weight averages three to four times that in the adult.