In a 2x2 design, after listing important personal reasons for smoking, 70 smokers were randomly told either that they had sufficient reasons for smoking (low internal constraint to change) or that they did not have sufficient reasons (high internal constraint to change) and were exposed to an anti-smoking message from a source with either expert (high external constraint to change) or non-expert (low external constraint to change) status. The main dependent variable was change in intention to give up smoking. The analyses revealed the predicted interaction between external and internal constraint: High internal constraint increased non-expert influence but not expert influence. Supplementary analysis showed that, when internal constraint was high, non-expert influence was related to the perceived quality of the message whereas when internal constraint was low, expert influence was related to the source's perceived motivation to inform, i.e. rather than to convince. These results were predicted on the basis of the link that targets establish in social influence settings between constraints to change that are internal (i.e. related to their personal beliefs, feelings or attitudes) and those that are external (i.e. related to the characteristics of the persuasive communication such as the status of the source). Copyright # 2002 John Wiley & Sons, Ltd.