The present study was conducted to obtain electrocardiographic data regarding the values of ST-segment morphology and duration from 84 apparently healthy Holstein cattle in fourteen different age groups from newborn to 7 years old. We recorded electrocardiograms in standard base apex lead system with the paper speed 25 mm/sec. and calibration of 10 mm equal to 1 mV. The morphologic assessments of electrocardiograms showed that all groups had normal isoelectric ST-segment, except newborns. Elevation of ST-segment was detected in 64.2% of newborn calves. The magnitude of this up sloping was 0.10±0.03 mV. The results of the comparison of ST-segment duration among different age groups showed that there were significant differences between the ST-segment duration of newborns (0.11±0.02, sec) and 5 years old (0.16±0.03, sec.), and also between 15 days (0.09±0.03, sec.) and 5 years old (P<0.05). The results of the present study may be accepted as normal ST-segment duration and morphology in different age groups of healthy Holstein cattle which will provide a good basis for judging and interpreting electrocardiograms.