When isolated salmon gills were perfused under simulated in vivo conditions, norepinephrine increased and calcitonin decreased the rate of flow of perfusate and the influx of calcium and phosphate through the gill. Only the increased water flow was mediated by β-adrenergic receptors. Differences in sensitivity to both hormones were observed with gills from prespawning and postspawning salmon; gills from freshwater-adapted fish were less sensitive to norepinephrine and more sensitive to calcitonin, suggesting a decreased production of this hormone in the late stage of the life cycle. The antagonistic effects of calcitonin and norepinephrine on the gill strongly implicate their regulatory role for calcium homeostasis in vivo; the major process being regulated is the influx of calcium into the gill. In fish, calcitonin performs an important role in regulating gill function.