This thesis is an investigation of the interactions between the estrogen (ER) and glucocorticoid (GR) receptors. The study examined the effects of ' in ovo' exposure of rainbow trout to GR and ER agonists (Cortisol and Bisphenol A [BPA], respectively) and antagonists (RU-486 and ICI 182,780, respectively) in various chemical combinations on mRNA transcription of 'ER[alpha], ER[beta], GR1' and 'GR2'. BPA did not affect mRNA transcription of 'ER[alpha]' or ' ER[beta]', and exhibited differential effects on 'GR1' and 'GR'2, depending on the time point at which gene transcription was analyzed. The antiestrogen ICI 182,780 did not act as a pure antiestrogen, but acted differentially on 'ER[alpha]' and 'ER[beta] '. Histological analysis revealed co-administration of cortisol and BPA 'in ovo' caused marked effects on notochord development indicated by an absence of the chordoblast layer. 'In ovo' exposure to toxicants can affect early epigenetic mechanisms, possibly leading to deformity and/or death of a developing rainbow trout embryo.