In recent years increased frequencies of malformations among eelpout embryos have been detected in monitoring programs in several Baltic countries [1,2,3]. Malformations can be induced in oviparous fish species by exposure to chemicals, including endocrine disrupting substances, in the laboratory. Eelpouts are the only viviparous fish in Northern Europe, which makes them very suitable for investigation of mother-offspring interactions and effects in the offspring upon maternal exposure to various chemicals. The causative agent or agents in underlying the malformations observed in the monitoring programmes are not known, but malformations upon exposure of pregnant eelpout to octylphenol and 17β-oestradiol (E2) have been observed at nominal concentrations of 500 ng E2/l and 100 µg octylphenol /l . Therefore, the aim of the present investigation was to establish no effect concentrations for the teratogenic effect of E2 in eelpout. We found that the highest concentration caused a significant increased frequency of abnormal larvae (P < 0,0001); 61,3% of the larvae in the exposed group displayed abnormal development compared to 14,7% in the control group. The majority of abnormalities observed were arrested development and spinal axis deformities. Moreover, it was observed that the amount of ovarian fluid was significantly reduced (P=0.001) in the highest exposure group. The study further showed that delaying the onset of exposure to the highest dose of E2 by approximately three weeks reduced the frequency of abnormal larvae to 2,8%. This suggests that timing of the exposure is crucial and that the embryos have a window early in development where they are sensitive to E2. Furthermore, it was noted that the measured concentrations are about 1/2-1/4 of the nominel concentrations. We will further investigate the fate of the added hormone in aired saltwater. Exposure of female eelpouts to environmentally realistic concentrations of 17β-oestradiol early in the pregnancy increases the abundance of malformations in the embryos.