This report dealt with the change of body temperature (rectal temperature) in the newborn cynomolgus monkeys (Macaca fascicularis) with a view to take it as an index for their health conditions. The body temperatures of 183 newborn babies which were well cared for by their mothers was 33.0 to 37.7 degrees C about 10 hr after birth. On the other hand, the body temperatures of 21 newborn babies which were not well cared for by their mothers was very low, ranging from 24.1 to 34.8 degrees C. In five newborn monkeys which were well cared for, the body temperature averaged about 36 degrees C just after birth and then declined rapidly by 32 to 33 degrees C at 40 to 50 minutes after birth. Then it gradually began to rise, reaching 36 to 37 degrees C at 180 to 240 min after birth. In the other four newborn monkeys which were delivered by Caesarean section, the temperature was 37 to 38 degrees C just after birth. Then it decreased to 29 to 32 degrees C at 120 minutes after birth when the newborns remained singly in a cage without warming.