Five unimmunized adult rhesus monkeys weighing 5.9-6.3 kg were challenged with a precalculated, inhaled dose of 20.95-41.8 micrograms/kg of aerosolized ricin. Two males and three females either died or were killed at the onset of respiratory distress between 36 and 48 hours post-ricin inhalation and were necropsied. Consistent gross and microscopic lesions were confined to the thoracic cavity. All monkeys had multifocal to coalescing fibrinopurulent pneumonia, diffuse necrosis, and acute inflammation of airways, and nearly diffuse alveolar flooding, with peribronchovascular edema. All monkeys also had purulent tracheitis, fibrinopurulent pleuritis, and purulent mediastinal lymphadenitis. One male monkey and one female monkey had bilateral adrenocortical necrosis. We attributed the cause of death to asphyxiation following massive pulmonary alveolar flooding. The lesions of acute inhaled ricin intoxication in rhesus monkeys closely resembled those lesions reported in rats with acute inhaled ricin intoxication.