This study was divided into two experiments. For the first experiment, heart rates during exercise of 21 healthy male dogs were recorded. The animals were brought to swim every 2 days, 8 times in all, for 10 minutes. Heart rates were measured every minute using a pulse watch. For the second experiment, 134 healthy adult dogs were categorized into three groups: small (41), medium (51) and large breed (42). Their heart rates were measured every minute for 34 minutes after the 5th swimming. In the first experiment, the heart rates during the 1st-4th swimming were significantly higher (p<0.05) than during the 5th 8th swimming. In the second experiment, the heart rates were significantly different (p<0.05) between small, medium and large dogs. No correlations were found between weight, age, and heart rate. From the results of the second experiment, we were able to formulate an equation for predictable heart rate of each group of dogs (small, medium and large dogs). From the results, we recommend that the limits on the length of time spent for aquatic exercise should be 15-30 minutes, depending on the breed (size) of dog. Moreover, there should be trainers observing each dog to prevent over-exercise.