Various man-made agents like pesticides, industrial chemicals and some natural substances have the potential to alter hormonal pathways that regulate reproductive processes in certain species of wildlife. Until now, only a few investigations have been undertaken to determine the effects of these substances on reproductive capacities (fecundity and fertility) in exposed invertebrate aquatic species. In this study one of the most abundant mollusc of European limnic systems, the hermaphroditic gastropod Lymnaea stagnalis was investigated to determine the effects of endocrine modulating substances on reproductive parameters. Known endocrine modulating substances were tested using the following nominal concentrations; Tributyltin (TBT in ng Sn/l) and b-sitosterol at 1, 10 and 100 ng/l, respectively, and 4-nonylphenol (4-NP) at 1, 10 and 100 mg/l. In addition, experiments were carried out with 1, 10 and 100 ng/l of t-methyltestosterone. All the testing was carried out on recently matured adults of Lymnaea. Fifteen to twenty snails per treatment were exposed for between 7 and 12 weeks in a semi-static test with a weekly exchange of testwater. Shell height and weight and mortality of the adults, egg production, hatching rate of the eggs, and histopathology of the adult snails were analysed. The same parameters were investigated on F 1 generation snails from exposed parents at two dates (1 week after exposure and at the end of the exposure duration). Treatments with TBT and 4-NP had only slight effects on the egg production of the adults and hatching rate of the eggs. However, increased histopathological changes were observed in epithelial tissues of the adult snails, e.g. lung and foot also characterised by extreme inflammatory processes. While b-sitosterol and t-methyltestosterone had no effect on the shell height and weight or the mortality of adult snails nor on the egg production or ensuing egg hatching rate, b-sitosterol treated snails presented a distinct atrophy of the albumen gland and so did t-methyltestosterone, albeit with a lower degree of atrophy. The observed histopathological effects due to exposure to tributyltin or 4-NP are suggested to lead to long-term adverse reproductive effects mediated by an impairment of the fitness of the snails. In the experiments the steroid-dependant (b-sitosterol and t-methyltestosterone) degeneration of the albumen gland caused no obvious adverse effects on the fecundity nor fertility of the adults or on F 1 -generation. However, the impact on fertility following a prolonged exposure to high concentrations of the phytoestrogen cannot be predicted.