Laboratory exposures indicate that estrogens and their mimics can cause endocrine disruption in male fishes. Studies of resident fish populations in estrogen-polluted waters support these findings, yet biomarker expression associated with exposure to estrogenic endocrine disruptors often differs dramatically between field and laboratory studies. The dynamic nature of aquatic ecosystems may be a contributing factor, as changes in environmental parameters can trigger physiological and anatomical changes in fish potentially altering the uptake and effects of estrogenic chemicals. To explore the role of environmental variables on biomarker expression in response to estrogenic exposure, adult male fathead minnows (Pimephales promelas) were exposed to estrone under varied environmental conditions (differing dissolved oxygen and chloride concentrations) in a laboratory flow-through system. Morphological and hematological parameters were assessed. At the concentrations tested, no main effect differences (P< 0.05) were found associated with dissolved oxygen (DO) or chloride treatments, excepting a decrease in secondary sex characteristic score (SCC) with low DO in one experiment. An interaction between DO or chloride and E1 was present in the case of blood glucose in one experiment. While vitellogenin concentrations were elevated with exposure to estrone (29 to 390 ng/L), the effect on other indices were variable. For SSC, blood glucose, hematocrit, and hepatic and gonado-somatic index – 1 of 4 experiments were affected by estrone exposure, while estrone treatment decreased body condition factor in 3 of 4 experiments. These results indicate the variable response to estrone, even within the confines of controlled laboratory conditions.