The effects of cyanobacterial hepatotoxins microcystin-LR, -RR, and -YR, of the neurotoxins saxitoxin and anatoxin-a, and of crude aqueous extracts of cyanobacteria were determined on the embryos of fish and amphibians from cleavage up to advanced stages of embryonic development. No acute toxic effects were observed after exposure to microcystins at 0.5, 5, and 50 g / L, but the timing of hatching was altered in rainbow trout: earlier hatching occurred with 0.5, 5, and 50 g / L microcystin-RR, 5 and 50 g / L microcystin-YR, and 50 g / L microcystin-LR; a minor delay in hatching was observed at ( 0.5 and 5 g / L microcystin-LR. In axolotl, a delay in feeding was observed microcystin-LR at 5 and ) 50 g / L; microcystin-YR at 50 g / L . After termination of exposure and rearing in toxin-free tanks, survival rate and growth were adversely affected in zebrafish larvae preexposed to microcystin-LR at 5 ( ) and 50 g / L. At the highest applied concentration of microcystin-LR 10 mg/ L , morphological effects were detected. Saxitoxin at 10 g / L and above delayed hatching in zebrafish and led to malformations and mortalities at 500 g / L. Hatching was also delayed in axolotl at 500 g / L saxitoxin. Anatoxin-a ( ) 400 g / L altered the heart rate in zebrafish, but no chronic effects were observed. Far more pronounced effects, compared to those obtained with pure toxins, were observed following exposure to various aqueous crude extracts of cyanobacteria from field samples and batch cultures: similar malformations combined with high mortalities and adverse effects on outer egg structures were observed concomitantly in all species. HPLC data showed that these effects cannot be attributed to microcystin content alone.