1. The rat testicle was used in studying the release of radio-labelled compounds from locally injected liposomes of various sizes, charge and lipid composition.
2. Large unsonicated liposomes markedly delayed the release of entrapped 125I-labelled albumin. Delay was due to liposomal entrapment rather than the presence of lipid per se and it was greater with neutral than charged liposomes. The albumin left the testis after release from, and not in association with, liposomal lipid.
3. Large unsonicated liposomes also delayed the release of entrapped actinomycin D and 5-fluorouracil. The former retained its cytotoxic activity and resulted in focal, dose-dependent tissue necrosis.
4. Small sonicated liposomes did not delay the release of entrapped 125I-labelled albumin, and enhanced release of actinomycin D, producing high concentrations of these compounds, which were released in association with liposomal lipid, in draining lymph nodes.