Abstract: In recent decades, data loggers and radio- and satellite transmitters have become an important technological part of research on free living animals. Loggers to track movements and behaviour are especially useful in seabird studies, as seabirds often travel considerable distances at sea where visual observations are challenging. The potential negative effects of these devices on mortality, behaviour and reproduction of birds have received some attention, but few studies have investigated the physiological effects of instrument attachment. In the present study, effects of global positioning system (GPS) loggers on black-legged kittiwakes Rissa tridactyla were investigated by obtaining behavioural and physiological parameters of stress (nest attendance, plasma levels of the avian stress hormone corticosterone [CORT], relative leucocyte counts, body mass and reproductive success) during 2 d of GPS-deployment. GPS-equipped kittiwakes had significantly elevated levels of CORT at recapture and also significantly extended the duration of feeding trips compared to controls. Kittiwakes with low body condition index (BCI) attended nests less than controls, and this pattern was more pronounced among GPS-equipped birds. The study underlines the need to take device effects into consideration when instrumenting seabirds. Potentially, effects may become more evident in birds with low body condition or in years where food is limited, and results from GPS-equipped birds should be viewed with this in mind.