We studied the therapeutic efficacy of an intravenously injected antifibrotic agent encapsulated in liposomes on inhibiting collagen accumulation in hypertensive blood vessels.cis-4-Hydroxy-l-proline (cHyp) in liposomes was injected into rats exposed to 10% O2, and drug effect was evaluated by measuring right ventricular pressure and hydroxyproline content of the pulmonary artery. Right ventricular pressure was 11 ± 1 mm Hg (mean ± SEM) 5 days after a single intravenous injection of 200 mg/kg cHyp in liposomes compared with 14 ± 1 mm Hg in rats injected with empty liposomes; hydroxyproline content was also reduced by cHyp treatment (87 ± 6 versus 107 ± 7 μg per vessel) (p<0.05 for both,n=6–9). Injections of cHyp in liposomes every 5 days partially prevented hypertension and vascular collagen accumulation during a 3-week exposure to hypoxia, and the dose required was one tenth the dose of unencapsulated cHyp. Therapeutic doses of cHyp in liposomes injected for 6 months affected tensile properties of main pulmonary artery and aorta, but there were no apparent histological effects on other organs. Liposomes injected intravenously were identified in pulmonary artery endothelial cells. The prolonged effect of a single injection of cHyp in liposomes may be due to uptake of the liposomes by the endothelium. Liposome delivery of drugs to the arterial wall may be useful in the study and treatment of hypertensive vascular disease. (Circulation Research1992;70:912–922)