This investigation was undertaken to determine values of electrocardiographic parameters in Spanish-bred horses at different stages of growth. The study was carried out on 179 healthy Spanish-bred (Andalusian) horses (98 females and 81 males), which were aged between 1 month and 17 years. The ECGs showed P waves of different configurations. The bifid shape deflection of the P wave was generally more frequent than the simple one in animals from 6 months of age. The first component of bifid P waves (P1) did not vary in a statistically significant way with increasing age, showing a mean value of 0.105 mV. The amplitudes of the P2 (the second component of the bifid P waves) and simple P waves showed statistically significant differences between the age groups. The highest values were found in animals up to 3 months and over 2 years of age. Several configurations of the T wave were observed in the different age groups: a negative shape was generally present for the youngest animals (foals up to 2 years old); the biphasic (/+) pattern became more frequent in animals 3 and 4 years old. In adult horses the positive configuration was the most frequent. No statistically significant differences according to age were found for the amplitude values of the different T-wave configurations.