Two generations of rats were raised at animal room temperature ranging from 12 to 32 degrees C at steps of 2 degrees C. The body weight of rats born in 22 degrees C environment and exposed to each temperature did not significantly differ within the range of 16-28 degrees C. The delivery rate, litter size, and weaning rate decreased at 30 degrees C and 32 degrees C, and only the weaning rate had a tendency to decrease at 12 degrees C. No significant difference was demonstrated within the 14-28 degrees C range for any reproduction parameter observed. The body weight of sucklings did not differ at birth within the 12-32 degrees C range; and at 3 weeks of age, there were no differences within the range of 18-28 degrees C. The body weight gain in both sexes after weaning was generally small when the temperature was below 18 degrees C or above 30 degrees C. There were no significant differences in food intake within the 20-26 degrees C range, in water intake within the 12-26 degrees C range, in hematological and serum biochemical values within the 20-26 degrees C range, and in organ weights within the 18-28 degrees C range. Therefore, the range of 20-26 degrees C (68-78 degrees F) was the optimum temperature range in rat rooms.