Ammonia can accumulate in high-density cattle accommodation during live export shipments and potentially threaten the health and welfare of the animals. The effects of 4 NH(3) concentrations, control (<6), 11, 23, and 34 mg/m(3), on the physiology and behavior of steers were recorded. The animals were held for 12 d under a microclimate and stocking density similar to shipboard conditions experienced on voyages from Australia to the Middle East during the northern hemispheric summer. In bronchoalveolar lavage samples, ammonia increased (P < 0.05) macrophage activity in proportion to NH(3) concentration and increased (P < 0.05) neutrophil percentage at 23 and 34 mg/m(3), indicating active pulmonary inflammation. Ammonia also increased (P < 0.05) lacrimation, nasal secretions, and coughing, particularly at 34 mg/m(3), indicating that the NH(3) was irritating the mucous membranes of the eyes, nasal cavity, and respiratory tract. Ammonia had no effect (P > 0.05) on hematological variables or BW. Twenty-eight days after exposure to NH(3), the pulmonary macrophage activity and neutrophil concentrations of the steers had returned to normal. It was concluded that ammonia concentrations of 23 and 34 mg/m(3) induced temporary inflammatory responses, which indicate an adverse effect on the welfare of steers.