Environmental pollutants are able to bind to the receptor for estradiol (ER) of cells and hence, have increased the interest for this hormone receptor. The aim of this diploma thesis is to asses whether or not the estrogen receptor α (ER α) is expressed in the cell line RTG-2. This cell line was established from gonads of the juvenile male and female rainbow trout (Wolf and Quimby, 1962) and has been extensively used in ecotoxicology. This cell line is used in our laboratory to establish a transcriptional reporter gene assay for estrogens and has also been used for transfections with a plasmid containing the cDNA for ER α. Both Stable transfected cells, that were generated and should contain overexpressed ER α, and the parental cell line RTG-2 were used in this thesis. To determine the expression levels of ER α protein, Western blot analysis was performed using a polyclonal antibody. We detected a faint band of the expected size of 65 kD in the control fish liver extracts, but not in the lanes containing the extracts from the RTG-2 cells. To investigate whether it was possible to detect ER α expression at the RNA level Northern blot analysis was performed. As probes DNA fragment of the rainbow trout ER α cDNA that encodes the DNA binding domain, or a fragment encoding the ligand binding domain of ER α were used. To control for equal amounts of RNA loaded on the gel the cDNA of the mouse a. actin that is expressed in most cell types has been employed. In Northen blots the a actin protein but not the ER α specific RNA was detected. By using a plasmid containing the cDNA for the second type of ER (ERβ, derived from a human probe) we detected a signal in the trout primary liver cell extracts and in the RTG-2 extracts. This signal was not detected in a probe containing the plasmid for ER α. We cannot contfirm the size of the mRNA since the ER α has not yet been cloned in rainbow trout. We conclude from our results that the ER a expression levels in this rainbow trout gonadal cell line is very low. If these cells express endogenous ER β at all, the levels are below the detection limits of the Norther blot technique.