The first part of the thesis reviews the possible aetiologies for idiopathic menorrhagia, discusses the investigation of patients complaining of abnormal menstrual bleeding in particular the role of outpatient management and summarises the medical therapies currently available, with emphasis on the synthetic progestagens. Evidence for the involvement of the endothelins, a family of powerful vasoconstrictors, and nitric oxide, a vasodilator, in endometrial homeostasis is reviewed. The structural changes seen in endometrium following exposure to synthetic progestagens are considered. A review of 400 patients referred for outpatient hysteroscopy and endometrial biopsy to investigate abnormal vaginal bleeding is presented and its clinical implications discussed. Results from a randomised comparative parallel group study comparing the efficacy of systemic and local progestagens is presented. 44 patients with objectively proven idiopathic menorrhagia were randomised to receive oral norethisterone or local progestagens via the levonorgestrel intrauterine system. Outcome measures included the change in menstrual blood loss after 3 cycles of treatment, side effect profiles and patient satisfaction with treatment. Endometrial biopsies were taken from patients participating in the above trial before and after treatment to examine the effects of exogenous progestagens on the endometrium. The following areas were investigated and are presented: changes in the distribution of endothelin A and B receptors: changes in the expression of endothelial and inducible nitric oxide synthase; the effects of exogenous progestagens on the lumenal epithelium using scanning electron microscopy and on morphological measurements of glandular and lumenal epithelium under light microscopy; alterations to the secretory function of epithelial cells using Dolichos biflorus agglutinin histochemistry, and to the stromal cells using α2 laminin, a marker of stromal cells which have undergone decidualisation.