Describes how a large ammonia release exposed about 150 nearby residents to this irritating gas and sent seven to hospital emergency rooms. Six weeks later the 41 most symptomatic people completed questionnaires and had physical examinations. The 12 most impaired had subsequent neurobehavioral testing to see if exogenous ammonia was toxic to the brain as is endogenous ammonia in hepatic coma. Spirometry was used to test pulmonary function. Frequencies of 35 symptoms were obtained by questionnaire, as were medical, respiratory and neuropsychiatric histories and data on previous chemical exposures. Comparisons were made with unexposed subjects after adjusting for age, sex, educational level and other determining factors. The exposed group’s mean values were significantly abnormal for simple and choice reaction, balance with eyes open, color discrimination, visual field performance in both eyes and hearing. Also abnormal were cognitive performance on Culture Fair, digit symbol, vocabulary and delayed but not immediate verbal recall. Making trails A and B was slow and fingertip number writing had excessive errors. Spirometric measurements were normal. Confounding features and biases were minimal. Exposure to ammonia, for a few minutes to several hours, was associated with neurobehavioral impairment measured after 22 months. Thus inhaled ammonia shares the toxicity of endogenous ammonia. Effects were persistent and are probably permanent.