While in the early 20th century, science occupied a privileged position in society and could ignore its critics, in the last 50--60 years, science's growing power has led to its increased social visibility and, subsequently, public scrutiny of its epistemological and ethical foundations. In this complex social context, national research institutions have to define their ethos against their external social environment.;In my dissertation, I draw on the rhetorical concept of ethos defined as an "appeal implicit in the speaker's character and credibility" (LaGranduer, 2003, p. 120) to analyze public ethos of two national labs, Brookhaven National Laboratory (BNL) located on Long Island, NY and Fermi National Laboratory (FNAL) located in the west suburbs of Chicago, IL. I argue that the labs' histories, their internal cultures, and their rhetoric are interrelated in significant ways. My data to investigate this claim come from (1) the interviews with the labs' public affairs and research staff, (2) study of media archival documents and (3) rhetorical analysis of the lab's on-line and print documents.;In order to provide a more nuanced analysis of the interrelationships revealed in these data, I analyze rhetorical strategies the labs use to develop their public ethos against the history of their relationships with the neighboring communities. I also explore the rhetoric the labs use on their websites to construct their on-line ethos with the neighboring communities and the interested publics. My analysis of the labs' public ethos, constructed through their on-line and off-line rhetoric, culminates in the examination of two very similar environmental risk situations (radioactive chemical leaks of tritium) that developed at both labs, but had dramatically different outcomes. I argue then that the difference in the outcomes of the two risk situations can be attributed to differences in the lab's public ethos and the models of public interaction/risk communication they used.;In conclusion I speculate on the implications of my study for science and rhetoric, and suggest projects for further exploration.