Changes in levels of extracellular noradrenaline (NA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindole-3-acetic acid (5-HIAA) in the frontal cortex, induced by exposure of unrestrained, conscious rats to novel environments, were compared using in vivo microdialysis. NA efflux increased when rats were transferred to a novel cage, but this was not significant when compared with either basal efflux or with changes after equivalent handling in their home cage. Increasing the intensity of illumination of the novel cage by fivefold significantly increased NA efflux with respect to basal efflux but not handled controls. However, a sustained and significant increase in NA efflux (cf. basal efflux or handled controls) was found when an unfamiliar conspecific was also present in the novel cage. In all cases, basal efflux was restored within 1 h of returning rats to their home cage. Neither handling nor environmental stimuli described above affected DOPAC efflux. 5-HIAA efflux was increased (cf. basal) in the presence of an unfamiliar conspecific, but this increase was no greater than that in handled rats. It is concluded that different naturalistic stimuli cause incremental changes in the levels of extracellular NA in the frontal cortex; these changes affect both phasic and tonic components of the response.