We indirectly tested the idea that the epithelial Ca²⁺ channel (ECaC) of the trout gill is regulated in an appropriate manner to adjust rates of Ca²⁺ uptake. This was accomplished by assessing the levels of gill ECaC mRNA and protein in fish exposed to treatments known to increase or decrease Ca²⁺ uptake capacity. Exposure of trout to soft water ([Ca²⁺] = 20 - 30 nmol l^-1) for 5 days (a treatment known to increase Ca²⁺ uptake capacity) caused a significant increase in ECaC mRNA levels and an increase in ECaC protein expression. The inducement of hypercalcemia by infusing fish with CaCl₂ (a treatment know to reduce Ca²⁺ uptake) was associated with a significant decrease in ECaC mRNA levels yet protein levels were unaltered. ECaC mRNA and protein expression were increased in fish treated with the hypercalcemic hormone cortisol but not by vitamin D3 (P = 0.059). Finally, exposure of trout to 48 h of hypercapnia (~7.5 mm Hg; a treatment known to increase Ca²⁺ uptake capacity) elicited an approximate 100-fold increase in the levels of ECaC mRNA and a significant increase in protein expression. Immunocytochemical analysis of the gills from hypercapnic fish suggested a marked increase in the apical expression of ECaC on pavement cells and a sub-population of mitochondria rich cells. The results of this study provide evidence that Ca²⁺ uptake rates are, in part, regulated by the numbers of apical membrane Ca²⁺ channels that in turn modulate the inward flux of Ca²⁺ into gill epithelial cells.