To investigate the respiratory response to exercise in patients with severe intermittent claudication, eight male patients, aged 57 years (range 43-73), with bilateral multi-segment atherosclerotic vascular disease, median maximum walking distance 50 m (range 20-200) and ankle-to-arm pressure index 0.4 (range 0.3-0.6), were studied before and after aorto-bifemoral bypass operation. Ventilation, CO2 output and O2 intake were recorded in the sitting position during 20 min of rest, 1 min of leg exercise on a bicycle ergometer [4.9 kJ (500 kpm)], and 20 min of recovery and rest. Before operation, maximal ventilation and CO2 output per minute were observed 2-4 min after cessation of work, while afterwards peak values were found during the work or the first minute of recovery. Pre-operatively, the extra ventilation and CO2 output during the work and recovery period and the recovery times of the ventilation and CO2 output per minute were markedly increased. Afterwards these values were clearly reduced towards normal. It is concluded that patients with severe intermittent claudication show a characteristic delay and prolonged rise in the respiratory response to exercise of short duration, which closely corresponds to the previously described pattern of outflow of hypoxia-generated metabolites from the exercising muscles. The pattern of respiratory response after operation reflects the fact that these patients also suffer from atherosclerotic heart dysfunction.