Intraperitoneal injections of 1,25(OH) 2 vitamin D 3 (1,25(OH) 2D 3) produced hypercalcemia in the marine teleost the Atlantic cod ( Gadus morhua) with increased plasma concentrations of ionized calcium (Ca 1), while the total plasma calcium concentrations (Ca T) were unaffected. A single injection of 10 μg kg bw -t of 1,25(OH) 2D 3 increased Ca I concentrations from 1.74 ± 0.03 m M to 1.8 ± 0.01 m M after 24 hr in Experiment 1 and from 1.67 ± 0.03 m M to 1.82 ± 0.06 m M after 72 hr in Experiment 2. This hypercalcemic effect was sustained by daily injections for 5 but not 7 days. Daily injections of a lower dose of 1.25(OH) 2D 3, 1 μg kg bw -1, caused hypercalcemia after 5 days (Ca I increased from 1.68 ± 0.01 m M to 1.76 ± 0.02 m M). Plasma calcium concentrations were not affected by any of the other secosteroids (vitamin D 3, (25(OH)), vitamin D 3 (25(OH)D 3), or 24,25(OH) 2 vitamin D 3 (24.25(OH) 2D 3), except for a decrease in Ca T concentrations after 7 daily in injections of 25(OH)D 3 (2 μg kg bw -1). Plasma phosphate concentrations were not changed by any or the seco-steroids. It is concluded that 1,25(OH) 2D 3 is hypercalcemic in the marine Atlantic cod.