SUMMARY: The effect of Cu2+ in concentrations from 1 × 10−7 to 1 × 10−4 mol/1 on water movement across the toad urinary bladder in vitro was studied. Although Cu2+ did not alter the permeability to water by itself, it strongly inhibited the hydrosmotic response induced by oxytocin and theophylline. This effect was present only when the ion was added to the serosal side. The observed inhibition was not completely reversible even at the lower Cu2+ concentrations. No interaction of Cu2+ with cyclic AMP was observed. However, a reversible inhibitory effect of the ion on the hydrosmotic response induced by increasing tonicity of the medium was demonstrated. In this case Cu2+ was effective from both sides. It is concluded that Cu2+ has two effects: (1) a non-reversible action on the cyclase system, inhibiting the responses to oxytocin and theophylline; (2) a reversible action on the 'osmosensitive mechanism', inhibiting the response induced by increasing tonicity.