It has been suggested that an algorithm of automatic adaptation of the AV delay to the instantaneous atrial rate be introduced into the program of DDD pacemakers to reproduce the physiological adaptation of the PR interval to effort, characterised by progressive shortening inversely linearly related to the heart rate. In order to evaluate the potential benefits in conditions of "standard" programming (basal AV delay the same for all patients: maximal frequency of 1/1 AV synchronisation uniformly limited to 120 bpm), a haemodynamic study was undertaken in 10 patients who had permanent DDD pacemakers implanted for advanced AV block. Measurements were taken during two standardized exercise stress tests (20 W/2 mn steps from an initial load of 20 W) performed in a random order, one with a fixed AV delay of 156 ms and the other with an "automatic AV delay" allowing linear reduction from a maximum value of 156 ms at rest to a minimum value of 84 ms at the maximum heart rate of 120 bpm. At the peak of exercise the "automatic AV delay" significantly affected 4 parameters: the paced ventricular rate (p = 0.008) and rate-pressure product (p = 0.005) which increased, pulmonary capillary pressure (p = 0.03) and cycle-to-cycle variability of systolic and diastolic blood pressures (p = 0.02 < p < 0.0001) which decreased. There was a tendency (NS) to slowing of the spontaneous atrial rate and to increase in cardiac output. This increase was significant in some patients and seemed to be due to a good relationship between the individual optimal value of the value basal AV delay measured by Doppler echocardiography and the value programmed in this study (156 ms).(ABSTRACT TRUNCATED AT 250 WORDS)