Abstract: The plasma cortisol concentrations in 60 normal dogs were determined by radioimmunoassay.According to the plasma cortisol values the dogs might be divided into 3 types,I.e.8:00 AM-low concentration with 5:00 PM-high concentration is called type A (30 dogs),8:00 AM-high concentration with 5:00 PM-low concentratiin is called type B (20 dogs),while 8:00 through 5:00 PM proximal concentration is called type C (10 dogs).We found that the circadian rhythms of the bio-o clock of plasma cortisol secretion in type B dogs were appeared in "U-form".There were two kinds of causation in these 3 type dogs:1)The plasma cortisol secretion of dogs were closely depended on the species of dogs.2)The plasma cortisol secretion of dogs were related with their envirements and their variant "lifeatyle".