NASA: The authors provide an overview of relevant results from endocrine studies in astronauts before, during, and after space flight. The hormonal systems examined are the water-electrolyte regulation, the hypothalamic-pituitary-adrenal axis, the hypothalamic-pituitary gonadal axis, the growth hormone-insulin like growth factor 1-prolactin system, hormones which affect bone turnover, the hypothalamic-pituitary-thyroid axis, and the endocrine pancreas. Hormones studied include renin, aldosterone, vasopressin, atrial natriuretic factor, cortisol, testosterone, lutenizing hormone, prolactin, growth hormone, insulin-like growth factor-1, insulin, glucose, T4, thyroid stimulating hormone, calcitonin, active D3, and parathyroid hormone.