Pituitary glands of sheepshead minnows, Cyprinodon variegatus, exposed for 19 months to 1-5 micrograms/l trifluralin were significantly enlarged and possessed histopathologic characteristics (when compared to glands of controls) such as pseudocysts, congestion of blood vessels and edema. Most of the fish with enlarged pituitaries also had diffuse vertebral hyperostosis and other dysplastic vertebral changes. Several speculative mechanistic paths are suggested for the mode of the effect of trifluralin on the vertebral and pituitary tissues. Study of the form and function of pituitary glands of teleosts from natural populations might provide indications of chronic physiological stress, particularly in relation to chemical pollutant stress.