A single population of Bar-tailed Godwits limosa lapponica was studied before departure from their wintering grounds on the Banc d'Arguin in Mauritania, and at a spring staging site in the Dutch Wadden Sea. On the Banc d'Arguin the godwits started gaining body mass in the course of March at rates of about 2.8 g d^-1 in ♂ ♂ and 3.2 g d^-1 in ♀ ♀, before leaving north between 25 and 27 April. In the Wadden Sea, body mass increased linearly over the entire staging period (29 April–31 May) at 5.6 g d^-1 in ♂ ♂ and 7.5 g d^-1 in ♀ ♀. Half of the mass increase was due to the deposition of fat, the other half to increases in fat-free tissue (mainly muscle protein). If the godwits flew at the heights with most tail wind assistance by varying their flight altitude up to 5.5 km, they gained an average wind assistance of 18 km h^-1, which would add considerably to their estimated air speed of 57 km h^-1. At an average ground speed of 57 18 = 75 km h^-1 the godwits would cover the required 4300 km in 57.3 h. During this flight, ♂ ♂ and ♀ ♀ were estimated to lose 136 g and 178 g respectively. Since half of these mass losses consisted of fat, they represent energetic equivalents of 3163 kJ and 3857 kJ respectively, leading to estimated flight costs of 55 kJ h^-1 for ♂ ♂ and 67 kJ h^-1 for ♀ ♀. The energy reserves stored on the Banc d'Arguin would not enable the Bar-tailed Godwits to cover the distance between W. Africa and the Wadden Sea in one flight without making good use of favourable high-altitude winds.