The average rate of swimming speed and the physiological status or stress of individual Atlantic cod Gadus morhua was monitored in response to short-term acute (STA) hypoxia (i.e. partial pressure of oxygen, reduced from 20·9 to 4·3 kPa within 1 h at 10°C). The STA hypoxic response of Atlantic cod was associated with a large primary increase (+29%) and a large secondary decrease (-54%) in swimming speed as well as major physiological stress (e.g. plasma cortisol = 214·7 ng ml^-1 and blood lactate = 2·41 mmol l^-1).