In the laboratory, fathead minnows were reared from time of hatching to adulthood at 5 concentrations of ammonia in a toxicity test lasting 304 days. The ammonia concentration test range was 9.36 to 30.0 mg NH3-N/liter (0.639 to 2.17 mg NH3/liter). Intracerebral lesions were observed in fish from each ammonia concentration but not in the controls. The severity of the lesions was positively correlated with increased ammonia concentration. Massive hyperplastic lesions developed in fish tested at the 2 highest concentrations. Histologically, lesions appeared to originate from the primitive meninx, which grew into the cranial cavity, and in some fish through the cranium. Based on the orderly appearance of tissues and evidence of regression of lesions, the lesions are not considered to be neoplasms but hyperplasias of primitive meninx tissue induced as a response to ammonia exposure. Fathead minnows (Pimephales promelas) have proved useful for acute toxicity testing of environmental contaminants and have a potential usefulness in carcinogenicity testing. They are easily maintained in the laboratory, can be readily induced to spawn during their first year of life, and have a relatively short period of embryogenesis.