A standardized Flack's test with continuous electrocardiographic recording was performed in 351 moderately trained athletes (317 men and 34 women; mean age 23.7 years). The test gave normal results in 310 subjects (88.3%). Tachycardia in excess of 100 beats/min was observed in 12 subjects (3.4%), and 56 abnormalities which were not present on previous ECG recordings at rest were noted in 29 subjects (8.3%); 27 of these abnormalities appeared immediately after a previous one. Out of these 56 abnormalities, 26 (44.6%) were associated with sinus dysfunction: sinus bradycardia in 8 cases, second degree sinoatrial block in 2 cases, third degree sinoatrial block in 3 cases and sinus arrest in 13 cases. In 16 cases of third degree sinoatrial block or sinus arrest escape rhythms were observed, including coronary sinus rhythm (3), junctional rhythm (12) and accelerated idioventricular rhythm (1). Five of the 56 abnormalities (8.9%) were disorders of conduction, i.e. first or second degree atrioventricular block, atrioventricular dissociation and blocked P waves. These disorders are known to be frequently associated with sinus dysfunction. In 9 out of 56 abnormalities (16.1%) disorders of excitability were present as atrial, junctional and ventricular extrasystoles. In 8 subjects the Flack's test resulted in the disappearance of disorders of conduction or excitability of the "athlete" type present at rest, as the Ruffier-Dickinson test, or the cycloergometric exercise test.(ABSTRACT TRUNCATED AT 250 WORDS)