Fasting is a common procedure for animals in experiments. Although fasting may be necessary for scientific reasons, it should be minimized. In the current study, jugular-catheterized male Sprague-Dawley rats in metabolism cages were fasted for 0 to 24 h before measurement of various physiologic markers (serum chemistry, CBC analysis, serum corticosterone). When controlled for cohort, rats fasted for 6 and 16 h had significantly lower serum glucose than did nonfasted rats. Other values did not differ from controls. Only rats fasted for 24 h had elevated serum corticosterone levels. Therefore, fasting for as long as 16 h has fewer effects on rats that does fasting for 24 h. Fasting for 24 h or more therefore should receive appropriate consideration by both scientists and the IACUC in the experimental design and the animal-use protocol.