Follicular fluid is composed partly of secretions from the follicle, and partly of exudates from plasma. Its composition reflects changes in the secretory processes of the granulosa layer and theca interna, and alterations in the components of plasma due to physiological or pathological processes. The presence of follicular fluid in so many species testifies to its potential importance in ovarian physiology, including steroidogenesis, growth of the follicle and ovulation, and maturation of the oocyte and its transport to the oviduct. The fluid can be collected easily, especially as the follicle enlarges, and there has been prolonged interest in its composition and functions. The source of the fluid in Graafian follicles was discussed widely in early debates, some authorities maintaining that it was produced within the follicle, whereas others believed it to be a transudate of plasma. Such a debate is now spurious, for various components of the fluid almost certainly