By screening for maternal qualities before puberty, it would be possible to detect and exclude gilts with a potentially high occurrence of risky behaviour with respect to early piglet mortality, and solely use potentially good mothers for reproduction. Thodberg et al. (1999) showed, that gilts’ reactivity in different stressful test situations at 4–4.5 months of age could predict maternal quality to some extent. In order to perform such a test under practical conditions it would be more efficient, though, if gilts of a wide age span could be subjected to the test. However, a non-detected change in reactivity with age entails the risk of selecting ‘‘suboptimal’’ gilts for reproduction. If gilts are to be tested at different ages, consistency of the behaviour trait used for selecting potentially good from potentially poor mothers has to be demonstrated. The present study aimed at investigating whether gilts’ reactivity in an arena test changes with age. The present study investigated whether gilts’ reactivity in an arena test, developed for selecting prepubertal gilts for reproduction, changed with age. One hundred and twenty gilts were used in the study of which 60 were tested at 3 months of age and re-tested at 4.5 months of age and 60 were tested only at 4.5 months of age. During the 3 min arena test the duration of inactivity was recorded by direct observation. The results from the present study showed that gilts were less inactive, when exposed to the arena test at 4.5 months compared to 3 months. This change in inactivity was due to an effect of age and not to an adaptation to the test situation since no difference was found between the gilts in the group re-tested at 4.5 months and the gilts in the control group, which had never been exposed to the arena test before. The measure of inactivity shows some consistency across time. Thus if a selection of prepubertal gilts is to be made, based on the arena test, the results of the present study show that the selection should be carried out within gilts tested at the same age.