Free [14C]cytosine beta-D-arabinofuranoside ([14C]ara-C) was completely absorbed from the peritoneal cavity of thoracic duct-cannulated rats by 6 hr after ip dosing. 14C levels in most tissues were higher at 4 hr than at 12 hr after dosing and were generally undetectable at 24 hr. By 6 hr after treatment only 2% of the dose was recovered in lymph, whereas 90% had been excreted in urine. Liposome entrapment of ara-C reduced the rates at which the drug was absorbed from the peritoneal cavity and excreted in urine while enhancing lymphatic uptake of the drug by more than 10-fold. Radioactivity in plasma and most tissues achieved higher concentrations and persisted for longer periods in rats given liposome-entrapped ara-C than in rats receiving the free drug. Most striking was the localization of 14C-activity in renal and thoracic lymph nodes of rats give liposome-entrapped ara-C, with 300 to 1000-fold higher levels present at 4, 12, and 24 hr after dosing than in corresponding lymph nodes of rats receiving the free drug. The metabolic conversion of ara-C to uracil beta-D-arabinofuranoside (ara-U) was reduced by approximately 3-fold following liposome entrapment of the drug. The enhanced lymphatic uptake and the localization and persistence of ara-C in lymph nodes resulting from liposome entrapment of the drug may be of benefit in treating tumors that metastasize via lymphatic pathways.