The possible coupling between regulation of the affinities for branchial Zn and Ca influx was investigated in juvenile rainbow trout Oncorhynchus mykiss acclimated to relatively hard fresh water ([Ca]=1.0 mmol l-1). The Km for branchial Ca influx was manipulated experimentally by exposing the fish to 2.3 micromol l-1 waterborne Zn for a total of 28 days. This procedure resulted in rapidly increased Km values for both Ca and Zn influx, an effect that remained through the experimental period. There was a significant linear correlation (r=0.88, P<0.02) between Km values for Ca and Zn measured at the same time points. Zn exposure caused progressively increasing maximum rate of transport, Jmax, values for Zn relative to the control value, but there was little, if any, effect on Jmax for Ca. These results support the idea of a shared transport site for Zn and Ca at the apical membrane of the gill epithelium and suggest that there is a certain degree of coregulation of branchial Zn and Ca uptake in rainbow trout. Removal of Ca from the water resulted in a large (six- to 24-fold) increase in affinity (decreased Km) for Zn influx and a modest (1.1- to 1.8-fold) increase in Jmax for Zn. Thus, Ca is a competitive inhibitor of Zn influx. In water lacking Ca, the Km for Zn in Zn-acclimated fish was no different from that of the control fish, suggesting that the Ca2+/Zn2+ transporter was regulated to improve Ca uptake.