Objective. To test the impact of a minimal, online planning intervention on physical activity in Australian office workers. Method. Employees were randomized to an implementation intention intervention (n = 124) or health information control group (n = 130). Measures of physical activity, past behavior, and motivation were taken at baseline and 6 weeks. Results. Analysis revealed both groups increased weekly physical activity sessions (intervention M = 1.12, control M = 0.78) at follow-up, but no significant difference was found between groups. Because the sample consisted of experienced exercisers, secondary analyses investigated differential effects for those who had lapsed over the previous year (nonmaintainers) and those who had maintained their previous level of activity (maintainers). For nonmaintainers, both planning and information provision successfully changed behavior, but only planning significantly increased physical activity in maintainers over the study. Conclusion. Different minimal intervention approaches may be useful for preventing long-term relapse and assisting people to improve regular exercise routines. The practical and theoretical implications are discussed.