Krohn and Zuckerman [1937], Fisher and Zuckerman [1937], and Aykroyd and Zuckerman [1938] have shown that the process of swelling of the sexual skin of immature female monkeys (M. nemestrina and M. mulatta) induced by oestrogenic stimulation is associated with the following occurrences: a general water-retention; increase in the size of the cells of the connective tissue of the sexual skin; the presence of a viscous exudate easily drained from cut surfaces of this tissue; variations of the water content of the tissue between cells and exudate; the exudate having a variable but high total osmotic pressure, which may exceed that of serum, and a total protein content of ca. 2·5 %. A further investigation of the exudate was undertaken in the hope that information might be obtained on the mechanism of sexual-skin swelling. Materials Immature female rhesus monkeys (Macaca mulatta) were injected with 100μg. oestrone in oil daily for varying