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Glucose-6-phosphate dehydrogenase (G6PDH), lactate dehydrogenase (LDH) and pyruvate kinase (PK) are key metabolic enzymes. G6PDH has been used as a biomarker of pollution-induced carcinogenesis in fish. LDH has been used as marker of lesions in toxicology and clinical chemistry, and PK catalyses the conversion of phosphoenol pyruvate to pyruvate, with regeneration of ATP. The effect of different concentrations of lead nitrate on the activity of these enzymes in two different early ontogenetic stages (embryonic and free embryonic stage) of the African catfish Clarias gariepinus was investigated. Embryo homogenates were used for measurements of G6PDH, LDH and PK activity spectrophotometrically at 340 nm and 25°C. The ontogenetic variations of the three enzymes during early ontogeny, from the 30 h to the 168 h post-fertilisation stage (PFS) (beginning of exogenous feeding), were studied. There was a significant decrease in activities of all three enzymes from 30 h-PFS to 96 h-PFS, followed by a significant increase in G6PDH and LDH. PK showed insignificant fluctuations in activity. Different patterns of enzyme activities were recorded due to exposure to different lead nitrate concentrations (100 μg/l, 300 μg/l and 500 μg/l). In the pre-hatching stage (30 h-PFS) the activity of the three enzymes increased at exposure to 100 μg/l lead nitrate and then decreased with increasing dose. In the post-hatching stages (48 h-PFS-168 h-PFS) G6PDH activity increased and LDH activity decreased with increasing lead concentrations. Unlike G6PDH and LDH, the PK enzyme fluctuated during the post-hatching stages and did not reveal a specific trend of response (increase or decrease) with increasing lead concentrations. Therefore, the measurement of G6PDH and LDH activities, but not PK activity, could be useful biomarkers of intoxication to reveal the embryotoxic potential of lead nitrate in fish embryos. The post-hatching stages of the African catfish are more sensitive than the pre-hatching stage (30 h-PFS) is, probably due to the protective capacity provided by the hardened chorion. The interaction and the main effects of age and lead doses were found to be highly significant, referring to the great impact of lead on these enzyme systems with increasing early development.