In addition to survival and hatching parameters, cytological alterations in liver and kidney of 4- and 6-d old zebrafish larvae (Brachydanio rerio) following single microinjection of fertilized eggs at the germ-ring stage with 5, 12.5, and 25 ng 4-chloroaniline/egg were investigated by means of electron microscopy. Whereas survival remained unaffected, microinjection with 4-chloroaniline disturbed hatching of larvae. Hatching was delayed by microinjection of 12.5 ng 4-chloroaniline/egg and above when compared to controls. Cytological investigations revealed ultrastructural changes in both liver and kidney in a dose- and time-dependent fashion. In the liver, major cytopathological changes included fenestration, fragmentation, and vesiculation of the rough endoplasmic reticulum, proliferation of atypical mitochondria, and atypical lysosomes. Furthermore, myelin whorls, lipid inclusions, and cholesterol crystals were increased, whereas glycogen stores were reduced. Renal tubular cells displayed altered brush borders, proliferation of nucleoli, atypical mitochondria, fenestrated, fragmented, and vesiculated RER cisternae, as well as giant lysosomes. Most of these effects indicate cellular dysfunction (e.g., disturbance of lipid metabolism in the liver), whereas others illustrate general cellular stress-responses to chemical aggression. Comparisons of results with those of previous studies based on conventional fish exposure prove the suitability and sensitivity of microinjection bioassays with zebrafish eggs as an alternative to conventional early life-stage tests.