SUMMARY A histological and histochemical study has been made of ovaries obtained from patients in early pregnancy, the duration of pregnancy varying from 6 to 20 weeks. During the first 10 weeks of pregnancy ovarian structure is virtually unchanged, compared with the post-ovulatory state. From 10 weeks onwards new Graafian follicles appear. These are limited in size, most achieving a maximum diameter of 4 mm. They are characterized by an early and excessive thecal development. Atresia overtakes these follicles and this occurs at all stages of development, suggesting a sudden and possibly recurrent change in gonadotrophic stimulation. Despite atresia the theca persists but histochemical tests suggest that its functional activity is limited. This loss of activity is associated with a recognizable histological change in the thecal cell. It is suggested that most of the alterations in ovarian structure are due to changes in the quality and quantity of gonadotrophins produced at this period of pregnancy.