Technical Abstract: The objective of the experiment was to determine the effectiveness of COWP in pregnant ewes and safety to lambs. COWP has been used recently as an anthelmintic in small ruminants because of the nematode resistance to chemical dewormers. A low dose COWP (< 4 g) used in Arkansas has been used in lambs without clinical signs of copper toxicity. Use in pregnant ewes has not been examined. Mature Katahdin ewes were administered 0, 2, or 4 g COWP 33 days before lambing in late March 2004. Lambs were weighed within 24 h after birth, at 30 and 60 days of age, and in mid-September (~120 d of age). Plasma was collected from lambs within 24 h after birth and at 30 d of age for determination of aspartate aminotransferase (AST). Within 7 d after COWP, FEC decreased by 1308 and 511 eggs/g (epg) in the 2 and 4 g group compared with an increase of 996 epg in the control group (P < 0.02). PCV was similar among groups between Days 0 and 35. At birth lamb plasma AST activity increased with increasing dose of COWP in dams (P < 0.001). At 30 d of age plasma AST activity was slightly greater in lambs from ewes treated with 4 g COWP (P < 0.02). Birth weights decreased with increasing COWP (P < 0.003). By 30 (COWP x birth type, P < 0.02) and 60 (COWP x birth type, P < 0.02) days of age, weight of multiple-born lambs decreased with increasing COWP. In mid-September (~120 d of age) weights of multiple-born lambs from ewes treated with 4 g COWP tended to be lightest compared with lambs from ewes treated with 0 or 2 g COWP or single-born lambs (P < 0.09). Lamb survival to 30, 60, or 120 d of age was not affected by COWP treatment to ewes. Administration of 4 g COWP to late pregnant ewes may negatively impact multiple-born offspring. Alternative parasite management may be necessary.