The effect of a copper deficiency on certain aspects of reproduction in ewes was investigated. An effective copper deficiency was induced by using the copper antagonists cadmium, calcium and sulphate. An average decline in the plasma copper concentration from 160 µg/dl to 56 µg /dl was achieved. Further indications of the effectiveness of the copper deficiency were the loss of crimp in the wool and the decline in the average liver copper concentration from 131 µg/g to 34 µg/g dry matter. Plasma copper and progesterone concentrations were determined on days 1, 5, 8, 12 and 16 (day of oestrous) of two consecutive oestrous cycles. A rise in plasma copper was found which reached a maximum at the time when the progesterone concentration was at its highest (day 12 of the oestrous cycle). A subsequent decline in the copper concentration followed and a minimum was reached at the time of oestrus, the period when progesterone concentration is at a minimum. Therefore it seems that the need for copper is higher for reproduction than for normal body maintenance.