Individuals often revise their belief in conditional relations when faced with contradictory evidence. However, individuals beliefs about the reliability of particular sources may influence their acceptance of such evidence. In three experiments, we examined effects of source credibility on belief revision. Participants were presented with a description of a mechanical system comprised of conditional relations with either uniform or randomly alternating components. Next, participants received a contradictory observation from a reliable, unreliable, or neutral source. When evidence came from an unreliable source, participants often failed to revise the conditional belief, regardless of the design of the system.