A number of studies have documented substantial loss of nuclear protein during aqueous nuclear isolation procedures. This loss can, to some extent, be counteracted by addition of impermeable macromolecules like polyvinylpyrrolidone, which prevent nuclear swelling. While nucleic acids appear to be much less susceptible to leakage during isolation, the effects of these additives on RNA release during in vitro incubation have not been examined. Here we show that addition of polyvinylpyrrolidone results in significant decreases in RNA transport; inhibition becomes maximal at 50-75 microM addition.