Research on effects of message repetition suggested an inverted U-shaped relationship between the number of message repetitions and the attitude toward the message content. Furthermore, it was indicated that when people are processing persuasive health messages, they use source credibility as a cue for judgments about the message. Built on that, repetition of an antismoking advertisement should initially increase source credibility and negative attitudes toward smoking. But when repetition rises to an excessive level, source credibility decreases, what in turn should decrease negative attitudes toward smoking. This study (N = 276) provided strong support for the hypothesized inverted U-shaped relationship. This effect was mediated by perceived source credibility and remained unaffected by smoking behavior. Implications are discussed.