In the present study the metabolic pathways during in vitro-fertilization of sea urchins were investigated, focusing on NADPH-producing enzymes. Additionally a fertilizationtest method was used to estimate the chronic toxicity of water samples and potentially harmful substances to marine organisms. A method for histochemical detection of enzyme activity in living cells during in vitrofertilization was developed and applied. Macros were designed for the application of the Image Analysis System in quantitative measurements of enzyme reactions. The activity of G-6-PDH, the key enzyme of the pentose phosphate pathway, was determined in kinetic experiments. The major activity of G-6-PDH was localized in the cortical region of the sea urchin oocyte. The results indicate that the enzyme is regulated by the concentration of NADP+. The active state of ovoperoxidase, which is responsible for crosslinking the fertilization envelope into a hardened matrix, could be demonstrated in unfertilized oocytes. Different embedding methods were tested to optimize the production of serial cryostat sections. The obtained sections show well-preserved morphological features of the sea urchin oocytes. The larval development of the sea urchin Psammechinus miliaris was observed in detail. Furthermore, the collected gametes were used for the analysis of water samples and pollutants using a fertilization test. The results show that salinity corrected water of the river Weser is not suitable for the use in aquaculture. A harmful effect of algal toxins on the fertilization of sea-urchin oocytes was observed.