The roles of photoperiod, energy balance, and concentrations of plasma total calcium (CaT) on daily changes in plasma calcitonin (CT) were investigated in broiler breeder hens (84-100 weeks old). In the first study, broiler breeder hens (n = 24), reared on 14L:10D, were divided into two groups. One group was transferred from a restricted diet (DR) of 150 g/day to ad libitum (AL) for 14 days, while the other group remained on DR. After 2 weeks of ad libitum feeding, birds from each group (AL and DR) were bled every 2 hr for 24 hr for measurement of plasma CaT and CT. In a second study, the hens (n = 20) were transferred to continuous light (LL) for 30 days. After the 30 days, food was removed from one group for 48 hr prior to blood sampling for 24 hr at 3-hr intervals. In a third study, birds were transferred to an ahemeral light cycle (11L:10D) for 28 days. Food was removed from the birds (n = 11) for 48 hr prior to blood sampling every 3 hr for 24 hr. Four weeks later the same birds were bled again for 24 hr, but this time the birds were maintained on a restricted feeding schedule. Plasma CT was measured by a specific heterologous electrochemiluminescent (ECL) assay while plasma CaT was measured by atomic absorption. The results showed that plasma CT concentrations did not correlate with plasma CaT concentrations. Comparisons made between initial and final CaT and CT concentrations indicated an effect of stress due to repeated handling of the birds. Concentrations of plasma CT were significantly reduced in the fasted animals (P < 0.05) compared to diet-restricted controls. There was a significant increase in plasma CT during the dark period of fed animals which was abolished in animals maintained on LL or fasted. In conclusion, a surge in plasma CT requires that the hens be provided food and that they be exposed to a dark cycle.