Genotoxicity assays were developed in embryonic stages of two species of fish, striped bass (Morone saxitilis) and sheepshead minnow (Cyprinodon variegatus), and in the adults of Fundulus heteroclitus using metaphase chromosome aberrations as the endpoint. Dose-dependent responses were obtained with several chemical mutagens, including 9-aminoacridine, ethylmethane sulphonate, cyclophosphamide and n-methyl-n-nitro-n-nitroguanidine, added to estuarine water at doses spanning several orders of magnitude in compound concentration. Exposures ranged from one to four days; however, two-day exposure time was found to be optimal in eggs and larvae. Tissues from the gills, kidney and intestinal tract of adult Fundulus were found to be responsive to mutagen exposure; however, the intestine gave the best responses. The results of these experiments suggest that these assays are sufficiently sensitive to be used in the field as well as in laboratory tests. (Copyright (c) 1988 Elsevier Applied Science Publishers, Ltd.)