1. Two experiments are reported. In both experiments a cereal-based diet containing 5 mg copper/kg was fed to two breeds of laying hens for 336 d. In Expt I four other groups were given this diet with the addition of CuSO4.5H2O to give added levels of 200, 400, 600 and 800 mg Cu/kg diet. In Expt 2 the levels of added dietary Cu used were 100, 200, 300 and 400 mg/kg.2. In Expt I records were kept of food intake, water intake, body-weight and egg production for eight 28 d periods and body-weight and egg number only were recorded for the full twelve periods. In Expt 2 full records, excluding water intake, were taken for all twelve periods.3. Food and water intake showed a quadratic response to level of added dietary Cu, being enhanced at lower levels and depressed at higher levels of addition.4. There was a quadratic response of total egg weight, mean egg weight and egg number to added dietary Cu. In Expt I egg number was maximum at 235 mg added Cu/kg diet for Warren Studler SSL (breed I) and at 170 mg added Cu/kg diet for Shaver 288 (breed 2). In Expt 2 no breed effect occurred, the maximum egg number being calculated to occur at 176 mg added Cu/kg diet.5. Depression of body-weight gain occurred at high lelvels of Cu addition. The depression of liver and oviduct weight found at high levels of addition appeared to be directly related to body-weight. A marked amount of feather loss also occurred at a high inclusion of CuSO4in the diet.6. The reproductive systems of the hens did not appear to be adversely affected at the levels of additive used. Gross and microscopic examination of specific tissues revealed no pathological effects although gizzard and intestinal weights were increased and caecal weight decreased by high levels of added Cu. Those aspects of the blood chemistry examined did not reveal any consistent effect between the two experiments.7. The liver Cu analyses indicate that between 600 and 800 mg added Cu/kg diet liver Cu concentration rises sharply. Both liver Fe and Zn concentrations showed a positive linear response to added dietary Cu. In the kidney Cu and Zn concentrations were increased but only to a limited extent, while the concentration of Fe was unaffected.