. 2000. Effects of a beta-adrenergic agonist (L-644,969) on performance and carcass traits of growing lambs in a cold environment. Can. J. Anim. Sci. 80: 459-465. A 2 × 2 factorial experiment was conducted to study the effects of the beta-adrenergic agonist L-644,969 (BAA, supplied at 0.28 vs. 0 mg kg -0.75 d -1 ) on growth performance, carcass traits and nitrogen balance of lambs at two environmental temperatures (0°C vs. 20°C). Sixteen Suffolk-Cross wether lambs (30.2 ± 2.06 kg BW) were randomly divided into four groups and exposed to each of four experimental treatments (20°C with control diet; 20°C with BAA-supplemented diet; 0°C with control diet; and 0°C with BAA-supplemented diet) for 5 wk. The collection period for nitrogen balance was 4 d during the 4th week of treatment. The low temperature decreased efficiency of feed utilization and increased backfat thickness by 20.8% (P < 0.01) and 35.1% (P < 0.05), respectively. L-644,969, on the other hand, increased daily gain, feed efficiency and hot carcass weight by 22.5% (P < 0.05), 17.2% (P < 0.05) and 9.6% (P < 0.05,) respectively. Dressing percentage (P < 0.01), biceps femoris weight (P < 0.01) and rib eye area (P < 0.01) were also increased, but abdominal fat expressed as a percentage of liveweight (P < 0.05) was significantly decreased by BAA treatment. The improved weight gain, muscle weight and ribeye area in response to BAA, and in the absence of an increase in total nitrogen retention, indicate that the responses are due to nutrient repartitioning. Temperature did not impair the nutrient repartitioning effect of BAA, but feed efficiency may be more improved by BAA for the animals in the cold environment. . D'autre part, le β-agoniste augmentait le GMQ, la valorisation des aliments et le poids de carcasse chaude, soit respectivement de 22,5, 17,2 et 9,6 %, tous trois significativement au seuil de P < 0,05. Une augmentation (P < 0,01) était également observée pour le rendement de carcasse, le poids du biceps femoris et la surface de la noix de côte ainsi qu'une diminution (P < 0,05) de la graisse abdominale en pourcentage du poids vif. L'amélioration du gain de poids, du poids musculaire et de la surface de la noix de côte causé par le β-agoniste ainsi que l'absence de tout accroissement de la rétention de N total portent à conclure que ces effets sont dus à une modification de la répartition des nutriments, effet qui ne semble pas avoir été inhibé par la température d'ambiance. Par ailleurs, le gain d'efficacité alimentaire attribuable au BAA serait plus prononcé dans le milieu froid.Mots clés: Performances, caractères de carcasse, β-agoniste, température, agneaux Ruminant animals including cattle and sheep often grow more slowly and have reduced efficiency of feed utilization in winter. The reduced efficiency has been attributed to limited intake, low digestibility, high maintenance requirement and the change of composition of liveweight gain (Christopherson and Kennedy 1983;Delfino and Mathison 1991;Webster et al. 1997