The fundamental aim of chronobiological analysis methods in the field of sport physiology in the horse is to establish the time interval in which the organism gives the best performance. Since it is well known that numerous physiological parameters (electrocardiographic, haemogasanalytic and haemodynamic) vary during physical exercise and influence athletic potential considerably, we studied the periodic variations in the following haematological parameters at a different frequency in 12 horses (6 athletes and 6 sedentary): erythrocytes (RBC), total leukocyte count (WBC), haemoglobin (Hb), packed cell volume (PCV), platelets (PLT), mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC). Some of the parameters studied showed periodicity at different frequencies both in athletic and sedentary horses: in athletic horses -infradian (RBC, PCV), circadian (RBC, WBC, PCV), ultradian (RBC, WBC, Hb); in sedentary horses-infradian (RBC, Hb, PLT), circadian (RBC, PCV), ultradian (RBC, Hb). The results obtained, therefore, can suggest that physical exercise is an exogenous synchronizer which modulates the periodicity of the haematopoietic system and of the functions of the organs involved in athletic performance.