Gill Ca2+ transport (GCAT) in fish is regulated by a number of different hormones. Stanniocalcin (STC) from the corpuscles of Stannius (CS) is an inhibitor of GCAT, whereas pituitary-derived prolactin and cortisol stimulate GCAT. Other than this, however, little is known about the effects of other hormones on this important transport process. The role of calcitonin (CT) in calcium homeostasis in fish is still controversial. Whereas many studies have shown significant effects of CT on plasma calcium levels, an equal number of studies have failed to find any correlations between plasma calcium and CT levels in fish. Previous in vitro studies have shown that salmon CT has potent inhibitory effects on GCAT in isolated, perfused fish gill preparations, a finding that has never been corroborated in vivo. Therefore, in this report we examined the effects of salmon CT on whole body 45Ca uptake (as a measure of GCAT) in young rainbow trout. In support of the in vitro findings, we found that CT had significant inhibitory effects on GCAT. In parallel studies, we found that CT had no effects on STC secretion and only modest, stimulatory effects on STC mRNA levels in cultured trout CS cells. These finding suggest that both CT and STC function as negative regulators of GCAT in fish.