Recent advances in molecular biology have provided new dimensions in the study of the reproductive system. There has been major progress in our understanding of the molecular mechanisms of hormone action in the past few' years. The symposium on "Molecular Basis of Reproductive Endocrinology" was organized to highlight new research findings on the regulation of the hypothalamic-pituitary-gonadal axis. The emphasis of the symposium was on physiological questions answered by the molecular biology approach. Studies on the functional relevance of gonadotropin releasing hormone and LH and FSH gene expression were presented, together with research on the molecular biology of ovarian and testicular steroidogenic enzymes and protein hormones. Also, several novel aspects of hormone gene expression in placental tissues were reviewed. The symposium was held July 25 to 26, 1991, immediately prior to the 24th Annual Meeting of the Society for the Study of Reproduction, on the campus of the University of British Columbia in Vancouver. Serono Symposia, USA generously financed and coordinated the meeting. We are indebted to Dr. Bruce K. Burnett and Dr. L. Lisa Kern for their professional assistance in the organization of the symposium. We would also like to thank Drs. Victor Gomel, Basil Ho Yuen, and John Challis, who served as session moderators. Most of all, we truly appreciate the efforts of all the invited speakers, poster presenters, and discussants in making this a memorable event as the largest one-day meeting of the Serono Symposia USA, series.