Individual differences in reactivity are relevant to animal welfare studies. Since reactivity refers not only to behavioural, but also to physiological responses, the latter could be used to assess such individual differences. Both changes in heart rate and in glucocorticoid concentration in plasma have been used with somewhat contradictory results. This may be due to the following causes: (1) differences in the exact methodology used, (2) use in the same experiment of animals reared in different environments and (3) use of physiological measures that may be affected by many different causes. It is suggested that although physiological measures may be used to assess individual differences in reactivity, particular attention must be paid to the exact methodology used. Furthermore, the results are likely to be more meaningful when animals reared in the same environment are used and when behavioural measures are also taken. © 1993.