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Dorper x Pelibuey ewe lambs (n= 44) initially weighing 24.56 +/- 0.70 kg were used to evaluate the effect of zilpaterol hydrochloride (ZH) on productive performance and some carcass characteristics in spring (n= 24) and summer (n= 20), under arid conditions of the Northwest of Mexico. In each season, ewe lambs were placed in individual pens and divided into two groups to be fed without ZH (SCZ) or 10 mg of ZH/animal/d (CCZ) for 32 d. Subsequently, all the ewes were slaughtered. The information was submitted to an analysis of variance under a randomized complete block design with a 2 x 2 factorial arrangement of treatments. The CCZ ewes of spring showed greater (P<0.01) final weight, daily weight gain and feed efficiency than CCZ ewes of summer or SCZ ewes of both seasons. The dressing in spring and summer was higher (P<0.01) in CCZ lambs than in those SCZ. Feed intake, fat thickness and kidney-pelvic-heart fat percentage did not change (P>0.05) by ZH effect; contrary, hot and cold carcass weights, and Longissimus dorsi muscle area were greater (P<0.05) in CCZ ewes than in SCZ ewes. Only feed intake and Longissimus dorsi muscle area were affected by season (P<0.01). In conclusion, high summer temperatures limit the ZH action to improve growth and feed efficiency of feedlot hair lambs, but not the carcass traits.