A field study tested Gollwitzer's (1993) proposition that the formation of implementation intentions specifying action opportunities accounts for the speed with which intentions are translated into action. Motivation to resume functional activity was assessed among a general population sample of patients prior to joint replacement surgery. At 3 months following surgery. people who had formed implementation intentions were found to have initiated 18 out of 32 activities sooner than people who had not formed implementation intentions. This difference could not be attributed to differences in motivation between the 2 groups. Evidence also showed that the formation of implementation intentions mediated the effects of behavioral expectation on speed of action initiation.