In the present study, the effect of copper was examined in the common goldfish (Carassius auratus auratus). Fish were fasted and exposed to either a high (0.84μM), a low (0.34μM) or a control copper concentration (0.05μM) for 1 and 7days. Swimming performance was not affected by either fasting or copper exposure. Food deprivation alone had no effect on ionoregulation, but low plasma osmolality levels and plasma Na(+) were noticed in fasted fish exposed to Cu for 7days. Both gill Na(+)/K(+)-ATPase and H(+)-ATPase activities were undisturbed, while both kidney ATPase activities were up-regulated when challenged with the high Cu levels. Up-regulated kidney ATPase activities likely acted as compensatory strategy to enhance Na(+) reabsorption. However, this up-regulation was not sufficient to restore Na(+) to control levels in the highest exposure group.