Embryo-toxic effects of lead nitrate of the African catfish with different modes of action were determined. Exposure of fertilized eggs to lead nitrate prolonged the incubation period and caused severe malformations. No specific response in the activities of the selected enzymes and no extremely DNA damaged was recorded in the pre-hatching stage exposed to the lead nitrate doses. In contrast, remarkable alterations in the activities of the same enzymes and significant dose-related DNA damage response over time in the post-hatching stages were recorded. Low susceptibility in pre-hatching stages is most probably a consequence of the chorion, which seems to protect the embryos from a range of external pollutants. In addition, histopathological malformations confirmed a decreased sensitivity in pre-hatching stages: The degree of malformations in the eyes, gills and notochord increased drastically after hatching. In our point of view, the protective capacity provided by a hardened chorion is a very interesting finding, well worth further studies. The selected biomarkers were sensitive detection methods for low-level toxicity of lead nitrate.