The efficacy of mixtures of narasin and nicarbazin were evaluated by comparing broiler performance, susceptibility to heat stress, and the mode of action against Eimeria. In a floor pen trial, narasin (70 ppm) alone or in combination with nicarbazin at levels between 10/10 and 50/50 ppm gave significantly better performance than unmedicated birds or birds given nicarbazin at 125 ppm alone. Amelioration of nicarbazin-associated mortality with heat as a stressor was observed in birds given the 50/50 ppm mixture of narasin and nicarbazin: mortality in these birds was similar to that of unmedicated birds and was reduced by 15 to 20% of that occurring in birds in the nicarbazin (125 ppm) treatment. The narasin/nicarbazin mixture (50/50) appears primarily to prevent further development of sporozoites. However, in birds treated with 25/25 ppm of narasin and nicarbazin, both the deleterious action of nicarbazin on merogeny and the antisporozoite activity of narasin were observed.