Male Japanese quail (n = 75) at 35 d of age were fed 20.0, 10.0, 5.0, 2.5 or 0 ml formalin (37% formaldehyde)/kg of their daily ration for 8 w. Quail fed 20.0 or 10.0 ml formalin/kg feed showed depression, decreased responsiveness, feed consumption, and body weights, and had vacuolation in the germinal epithelial layer of their seminiferous tubules. Formalin feeding at up to 5 ml/kg was associated with decreased weight of testes, and up to 2.5 ml/kg feed resulted in smaller diameter seminiferous tubules.