This paper demonstrated two modelling techniques and three interpretations for the inclusion of past behaviour in a theory of planned behaviour (TPB) framework using structural equation modelling. Model 1 examined past behaviour as either a causal influence or as an autoregressive influence on current behaviour. Model 2 demonstrated a novel approach to including past behaviour and current behaviour while preserving the tenets of the TPB, as it freed the residual correlation between past and present behaviour but not the causal path. Participants were 305 undergraduate students (mean age = 19.42 years) who completed measures of the TPB, past exercise behaviour and current exercise behaviour at one-month follow-up. Results demonstrated the importance of a priori conceptualization of past behaviour, as the models provide differences between estimated coefficients. Subsequent interpretation and decision to use each modelling strategy depends on the research objective and theory.