The present data reveal that (a) the Stannius corpuscles (CS) of Ompok bimaculatus are active even in their natural freshwater environment (0.4 mM Ca2+/liter) and produce hypocalcemic hormone or stanniocalcin, which lowers the Ca level in normal freshwater fish injected with 0.2 ml of CS extract (1 mg of CS) per fish, (b) this hypocalcemic activity is enhanced in fish that are adapted to a calcium-rich environment (0.6% CaCl2 solution), and (c) adaptation to this calcium-rich environment leads to hypercalcemia in fish. Administration of the extract (from fish previously reared in 0.6% CaCl2 medium) to hypercalcemic fish leads to severe hypocalcemic conditions. It is suggested that the CS of O. bimaculatus produce a general hypocalcemic hormone which is active in the natural freshwater environment (with 0.4 mM Ca2+/liter) and whose activity is enhanced under hypercalcemic conditions after adaptation to a calcium-rich habitat.