It is well-known that Trichinella spiralis larvae can live for an extraordinarily long time encysted in the muscles of a host. There are many reports in the literature describing larvae which have been found alive in human muscle, for from 5 to 30 years after the disease was thought to have been contracted (1, 2, 3, etc). In the authors' experience, numerous living larvae were found in a muscle removed at biopsy from a man who had had a very severe attack of trichiniasis seven and one-half years previously. This man had had the cause of his illness explained to him and had been so afraid of contracting the disease a second time that he claimed that he had not eaten meat of any kind since. It seems probable, therefore, that the larvae found in this case must have been present since the original illness. The generally accepted view, as expressed many years ago by Staubli (4), is that, after encystment, Trichinella larvae enter a latent stage in which they are walled-off from the tissues of the host. It is true that the larvae do not grow or develop while encysted, but it is well-known that they are capable of movement within their cysts. Stannard, McCoy and Latchford (5) showed that larvae digested free from their cysts exhibit both an aerobic and an anaerobic metabolism when kept in Tyrode's solution at 37? C. It is not known whether encysted Trichinella larvae live on food materials stored within their own bodies, or whether they absorb nourishment through the cyst wall from the host. In view of the long periods of time during which they remain viable in the encysted stage, the latter seems the more probable. The present preliminary experiments are designed as a test of the permeability of the cyst wall. Within the past few years, the production of radioactive isotopes of such elements as sodium, potassium, phosphorus, iron, etc, has afforded the means for tracing the distribution of these substances in the bodies of experimental animals. This method of study, of course, is equally applicable to parasites which may be present in these animals. In the present experiments, tests were made of the ability of encysted Trichinella