The effects of elevated dietary calcium (as CaCO3) and acute waterborne Cd exposure (50 microg/l) on whole body uptake, tissue uptake, and internal distribution of newly accumulated Cd, Ca2+, and Na+ in juvenile rainbow trout were examined. Fish were fed with three diets (mg Ca2+/g food): 20 (control), 30 and 60 for 7 days before fluxes were measured with radiotracers. The highest dietary Ca2+ elevation reduced waterborne whole body Ca2+ uptake, but did not protect against inhibition of waterborne Ca2+ uptake by waterborne Cd. Both Ca2+-supplemented diets reduced newly accumulated Ca2+ in the gills in relation to the control treatment, but did not prevent the Cd-inhibiting effect against accumulation of new Ca2+ in most compartments. Fish fed with Ca2+-supplemented diets showed markedly lower rates of whole body uptake and internalization (in some tissues) of waterborne Cd, illustrating that, while dietary Ca2+ supplementation did not protect against the impact of waterborne Cd on waterborne Ca2+ uptake, it did protect against the uptake of Cd. Waterborne Cd had no effect on Na+ fluxes, total Cl-, and in most body compartments, newly accumulated Na+ and total Na+ were also not affected. Dietary supplementation with CaCO3 had the same protective effect as demonstrated by dietary supplementation with CaCl2 in an earlier study. Thus, the reduction of waterborne Cd uptake and internalization by dietary Ca2+ was specifically due to Ca2+ and not to the anion.