The present study shows that for a personally relevant counterattitudinal issue, a highly credible source can alter persuasibility by increasing a subject's message-relevant thinking. Previous failures to show this effect were probably due to the highly thoughtful nature of typical research subjects, when confronted with involving issues. In the present study, field-dependent and field-independent subjects heard convincing or refutable counterattitudinal speeches given by sources of high or low credibility. Results indicated that subjects who are typically low in differentiation of stimuli (field-dependent subjects) showed differential persuasion to strong and weak arguments only when they were presented by a highly credible source. For subjects who are typically high in propensity to differentiate stimuli (field-independent subjects), the arguments were differentially persuasive for both high and low credible sources. These results are consistent with the hypothesis that increasing source credibility can enhance message-relevant thought for subjects who typically do not scrutinize message content.