This paper considers three potentially important modifications to the theory of reasoned action (Fishbein, 1980). It was hypothesized that behavioural norms, or beliefs about the behaviours of others, are important influences above and beyond subjective norms; the effects of attitudes and normative beliefs on intentions and behaviours are interdependent and interactive rather than additive; and the beliefs underlying subjective and behavioural norms are multidimensional rather than unidimensional. These hypotheses were tested in two surveys of smoking intentions and behaviour. The respondents in the first study were primary school children and those in the second study were college students. In both cases behavioural norms and the attitude-normative belief interactions led to significant increases in the prediction of smoking intentions and behaviour. Exploratory factor analyses also suggested that the beliefs underlying subjective norms may be multidimensional rather than unidimensional. These results thus support the hypotheses and suggest that the theory of reasoned action should be modified accordingly.