1. The male rat is more susceptible to infections of Nematospiroides dubius than the female. As the rat grows older the resistance of the female rat to infection increases at a greater rate than that of the male.
2. The course of the infection is modified by the sex of the host.
3. More larvae penetrated the intestinal mucosa to encyst in the male than in the female. More larvae, however, formed cysts in the female than in the male rat by the fifth day.
4. The male harboured more adult worms than the female rat, although this difference was not significant in the immature animals.
5. The sex resistance of the rat to N. dubius infections was removed by bilateral gonadectomy. Castration decreased the susceptibility of the male rat, while spaying increased it in the female compared with the susceptibility in the respective normal hosts.
6. Subsequent replacement of the homologous sex hormone in the gonadectomized rat restores the sex resistance, and may even increase it (particularly in the immature animals). Oestradiol increased the resistance of the spayed female rat, while testosterone increased the susceptibility of the castrate male rat to infection.
7. Oestradiol implanted in castrate male rats increased the resistance of these hosts to a greater level than was shown in the normal male rat.
8. The rat shows a marked age resistance over which the sex resistance is superimposed.
9. The relationship between the sex of the host and its resistance to infection is discussed.
This work was done during the tenure of a Department of Scientific and Industrial Research Studentship. My thanks are due to Dr E. T. B. Francis for his helpful and critical supervision and to Professor I. Chester Jones, in whose department the work was done, for the facilities he provided.