Positive liposomes consisting of phosphatidylcholine, cholesterol and stearylamine and negatively charged liposomes consisting of phosphatidylcholine, cholesterol and phosphatidylserine, were double labelled with either 3H-labelled dipalmitoyl phosphatidylcholine and [14C]cholesterol or with [14C]cholesterol and [3H]methotrexate entrapped in the aqueous phase. The plasma levels and urinary excretion of radioactivity from sonicated and non-sonicated liposomes were then compared with the levels of radioactivity from free [3H]methotrexate during a 4 h experimental period after an initial intravenous injection in cynomolgous monkeys. Tissue uptake at the completion of the 4 h experimental period was also measured. It was found that plasma radioactivity from [3H]methotrexate and [14C]cholesterol in sonicated positive liposomes was cleared more slowly than from comparable non-sonicated liposomes, and considerably slower than from free [3H]methotrexate. Radioactivity from sonicated negative liposomes was cleared more rapidly than from positive sonicated liposomes. Positive liposomes captured considerably more [3H]methotrexate than negative liposomes and showed very low permeability to [3H]methotrexate in in vitro studies, even in the presence of high concentrations of serum. [14C]Cholesterol radioactivity was cleared more rapidly from plasma than 3H-radioactivity from liposome-entrapped [3H]methotrexate for double-labelled sonicated liposomes and generally showed greater uptake into tissues and red blood cells. 3H-labelled dipalmitoyl phosphatidylcholine in sonicated positive liposomes was cleared faster than [14C]cholesterol during the first 3 h. The more rapid disappearance of [14C]cholesterol from the plasma was complemented by greater uptake into a number of tissues, and positive non-sonicated liposomes were taken up to a greater extent by the spleen than equivalent sonicated liposomes. Renal excretion of 3H from liposome-entrapped [3H]methotrexate was considerably less than that of 3H from free [3H]methotrexate. There was insignificant excretion, however, of 14C from cholesterol in the urine. Entrapment in liposomes completely prevented the otherwise considerable breakdown of free methotrexate to 3H-containing products in plasma and partially prevented its breakdown in tissues. These studies indicate marked differences in the distribution of liposomes in vivo due to surface charge and size, and some degree of exchange of the lipid components of the liposome bilayer independent of the distribution of the entrapped species. They also show that entrapment in liposomes can reduce metabolic degradation of a drub, maintain high plasma levels and reduce its renal excretion.