The purpose of this study was to determine the specific expression profile of 132 genes, some of which are estrogen responsive, in largemouth bass (LMB) following exposure to estradiol (E(2)), or to two hormonally active agents, 4-nonylphenol (4-NP) and 1,1-dichloro-2, 2-bis (p-chlorophenyl) ethylene (p,p'-DDE), using gene array technology. The results of these experiments show that LMB exposed to E(2) and 4-NP had similar, but not identical genetic signatures for the genes examined, some of which are known to be estrogen-responsive genes. The differences suggest that 4-NP may have additional modes of action that are independent of the estrogen receptor (ER). We have also shown that exposure of male LMB to p,p'-DDE results in an increase in some estrogen-responsive genes. But in female LMB, the observed changes were a down-regulation of the normally up-regulated estrogen responsive genes. Other genes were also down-regulated. These results suggest that p,p'-DDE may affect regulation of genes differently in male and female LMB. This study further suggests that gene arrays have the potential to map out the gene activation pathways of hormonally active compounds.