Lake sturgeon, Acipenser fulvescens, are one of a few species of cartilaginous fishes that complete their life cycle entirely in freshwater. Sturgeons maintain very low concentrations of circulating calcium (Ca(2+)) compared with other vertebrates, and therefore, face unique challenges in regard to Ca(2+) regulation, which are likely to be magnified during vitellogenic stages of the reproductive cycle. In the present study, Ca(2+) concentrations and associated hormones of female and male lake sturgeon were examined in two wild populations, and were related to reproductive stage. In both populations, free, bound and total Ca(2+) were low, peaking in mid-late vitellogenic females. Internal Ca(2+) and phosphate (PO(4)(3-)) concentrations were inversely related to environmental concentrations, suggesting that these ions are preferentially retained and that mechanisms for mobilization are up-regulated under diminished environmental concentrations. Plasma 17beta-estradiol, 11-ketotestosterone and testosterone, peaked in mid-late vitellogenic females, while the androgens peaked in spawning males. Urine Ca(2+) was more tightly regulated than other divalent ions and decreased in spawning fish. Therefore, the increases in free plasma Ca(2+), the very low circulating concentrations of free and total Ca(2+), and the increase in PO(4)(3-) and bound Ca(2+) in low Ca(2+) environments indicate unique adaptations to Ca(2+) regulation in the lake sturgeon.