Pelibuey and Katahdin are two the most extensively used hair sheep breeds in tropical regions characterized by high temperatures and humidity. Lactation influences profitability of sheep in dairy and non-dairy production systems. Therefore, the aim of this study was to evaluate the effect of breed and litter size (LS) on milk yield and milk composition of Pelibuey (n = 28) and Katahdin (n = 21) ewes reared under tropical conditions of Mexico. Milk, fat, protein and lactose yields, fat and energy-corrected milk, and fat, protein and lactose percentages were measured for seven weeks. Body weight (BW) of each ewe was measured weekly. Breed effects were significant (P < 0.001) favouring Katahdin breed on all yield traits. Ewes rearing two lambs had higher yields (P < 0.01) for all traits than ewes rearing one lamb. Fat percentage and BW were significantly (P < 0.05) affected by a triple interaction effect of Breed x LS x Week. So, important differences in milk yield and composition among breeds and LS were shown and might be considered in order to improve profitability of sheep production systems under tropical conditions.