Honeybees (Apis mellifera L.) will be trained to fly from their colonies to artificial flowers and the doubly-labelled water technique will be applied to measure energetic expenditure during free flight. 1) Flight distance will be varied to separate flight costs from total energetic expenditure and 2) artificial weights will be attached onto bees to examine the influence of increased flight load on the flight costs. Measurements of energetic costs of insect flight have so far been restricted to laboratory experiments. This has particularly limited the range of conditions under which flight performance and flight costs could be analysed and generally left the question open to what extend laboratory data can be used to estimate field metabolic rates. Recent validation of the DLW method for bumblebees (Wolf et al. 1996) and honeybees (Wolf, in prep.) makes it now possible for the first time 1) to extend flight cost analysis to flight speeds which could not be tested in laboratory experiments (Ellington et al. 1990) and 2) to test the validity of results of artificial loading experiments (Wolf et al.1989)