This Test Guideline is used in the assessment and evaluation of the toxic effects of organophosporus substances. Daily doses of the test substance are administered orally (preferably by gavage or administration of gelatin capsules) to domestic laying hens (aged 8 to 12 months) for 28 days. The animals are observed at least daily until 14 days after the last dose. Biochemical measurements are undertaken on hens randomly selected from each group after the last dose. Two weeks after the last dose, the remainder of the hens are killed and histopathological examination is undertaken. The treatment group should contain at least 12 hens. Generally, at least three treatment groups should be used. The highest dose level should be chosen with the aim of inducing toxic effects; thereafter a descending sequence of dose levels should be selected. A limit test may be performed if no effects would be expected at a dose of 1000 mg/kg bw/d. The results of this study include weighing at least once a week, biochemistry (neuropathy target esterase, acetylcholinesterase) and, at least, and detailed observations, as well as gross necropsy and histopathology. The findings of this study should be evaluated in terms of the incidence, severity, and correlation of behavioral, biochemical and histopathological effects and any other observed effects in each of the treated and control groups.