The precordial leads (CV6LU,CV6LL,CV5RL,V10) were used for taking E.C.G from 100 Iranian German shepherd dogs, for determination of heart rate, different wave forms (P,QRS,T), intervals and voltages. Results showed that in the most dogs sinus arrhythmia was the normal rhythm. Heart rate was significantly different between dogs more thanl2 movths and less than 12 momth of age,but no significant difference was observed between male and female dogs. The P wave was positive in CV6LL and CV6LU. In lead V10, 81/8% was isoelectric and 11/1% positive and 7/1% was negative. The QRS complexes in all leads were different in shape. The P.R interval between dogs above 12 months and under 12 months of age in leads V10 ,CV6LL was
significantly different. Mean Q-T interval was also significant in leads CV6LL,CV6LU. Mean height of S wave in CV5RL was significantly different in dogs younger than 12 months compared with dogs older than 12 months of age being higher the latter group.