Four well trained men crossed the inland glacier of Greenland in 1988, retracing the route of the famous arctic explorer Fridtjof Nansen from 1888. They used true copies of Nansen's equipment in all details. Each pulled at sled weighing 130 kg at the start. The expedition lasted 42 days, covered 500 km, highest point 2800 m above see level, coldest temperature -50 degrees C. Serum testosterone, sex hormone bindings globulin (SHBG), luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactine (PRL) were determined at weekly intervals, before, during and after this exceptionally long lasting, physically exhausting and stressful exercise. Testosterone fell to a mean level of 1.8, 2.8 and 3.8 nmol/l during the last two weeks of the expedition (p less than 0.001), and SHBG increased correspondingly from 26.7 nmol/l before start to more than 50 nmol/l (p less than 0.05). PRL levels were significantly decreased (p less than 0.005) at the time point when testosterone was maximally reduced. The normalization of testosterone following the expedition was associated with a significant increase in LH (p less than 0.05). In spite of low testosterone levels, lean body mass increased during the expedition in 3 of the participants.