Behavioural baseline is a critical determinant of response to drugs and other manipulations. In the present study, the influence of several organismic and procedural variables on basal plus-maze profiles in mice were examined. The methodology incorporated traditional behavioural parameters as well as novel measures derived from ethological analysis. Experiment 1 showed that social isolation for 1-3 weeks enhanced aggression in male DBA/2 mice but did not substantially alter their behaviour on the maze. A reduction in stretch attend postures did, however, suggest a minor reduction in anxiety in socially isolated animals. In Experiment 2, males of both DBA/2 and T1 strains exhibited higher levels of general activity on the maze than their female counterparts. Although additional evidence suggested that DBA/2 (but not T1) females were less anxious than males, no major sex differences were noted. Experiment 3 revealed a significant strain difference in plus-maze profiles, with T1 males showing a lower basal level of anxiety than DBA/2 males. This study also demonstrated that DBA/2 and T1 males react very differently to prior novelty experience, with enhanced anxiety evident in the former and reduced anxiety in the latter. Together, these findings point to a range of organismic and procedural variables that may account for inconsistencies in the literature on the elevated plus-maze.