16 racing pigeons were exposed to a 12h light : 12h dark cycle under the conditions of individual housing and free access to food and water. In two experimental parts I (six pigeons) and II (ten pigeons) the ECG was registered for several days by the means of an electrocardiograph or a radio telemetry system. In part I, a diurnal sinusoidal rhythm of the resting heart rate could be shown in six clinically healthy pigeons. The mean heart rate of all birds was 121 bpm (beats per minute). The amplitude of the mean best-fitting cosine function was 17 bpm, the acrophase was at 5 hours, 20 minutes after the beginning of the photoperiod. The rhythm´s most important Zeitgeber appeared to be light or rather the time of onset and duration of the photoperiod. A separate influence of the feeding schedule on the heart rate´s rhythm could not be shown. Further possible influences are discussed. The diurnal rhythm reflects the fluctuation of autonomous activity during a 24h span. During the dark period there is a parasympathetic prevalence on the sinus node activity, during the light span the vagal tone decreases. All but one of the 10 pigeons of experimental part II had shown second-degree heart (atrioventricular, AV) blocks in an electrocardiographical preexamination. In the main investigations, a diurnal rhythm of the resting heart rate was found in all birds. The mean heart rate was 112 bpm. In none of the pigeons a correlation between heart rate and percentage of AV blocks could be observed. Unanimously a peak of second-degree heart blocks appeared between 5 and 7 a.m (dark span). One pigeon with a high percentage of heart blocks of atrial rate (up to 27%) expressed an AV block rhythm with a period of 12 hours. In all pigeons the AV blocks disappeared with an increase in heart rate, indicating a relationship between blocks and high vagal tone. According to the current opinion the lack of correlation between heart rate rhythm and heart blocks might be due to the inability of the sinus node to depress its activity appropriately to the atrioventricular conduction time. Additionaly, a new hypothesis is postulated saying that there might be different vagal fibres innervating sinus node and AV node. They might be controlled individually by the CNS and might therefore lead to the observed divergence of the rhythms. In contrast to the results of the preexaminations, two pigeons did not show AV blocks anymore. In some pigeons an inrease in heart blocks during paradoxical (stress) situations and before the daily supplementing of food was found. Possible explanations for these observations are discussed. Supraventricular premature contractions were noticed in three pigeons. One of them expressed a rhythm of premature contractions with a peak between 5 and 7 a.m. During this time there seems to be a state of autonomic instability supporting the formation of premature contractions. Another pigeon regularly showed sinoatrial blocks with a tendency to a diurnal rhythm. Two birds had paroxysmal tachycardia. Again a causal instabilitiy of autonomic activity is discussed. AV dissociation was observed in one pigeon. Possible pathologic causes of the arrhythmic events are discussed in connection with the results of the clinical examination. Although they seem to be unlikely they cannot be excluded completely.