Serum calcitonin (CT) was measured in male and female Fischer fasted and fed rats from 1-24 months of age. Previous findings by ourselves and others that serum CT rises with advancing age and is higher in females than in males were confirmed and extended. The effects of feeding were examined with the use of three different protocols, overnight fasting then feeding for 1 h, feeding ad libitum, and feeding rats for 2 h after they had been on a fixed feeding schedule for 4 weeks. At 1 month of age serum CT was too low to measure in either sex, fed or fasted. From 2-24 months of age, feeding increased serum immunoreactive CT much more in females than in males. The increase in serum CT was accompanied by a slight decrease in serum calcium. We concluded that the release of CT in rats after eating is sex related.