The peripheral-type benzodiazepine binding site (PTBBS) ligand, PK 11195, is known to be a marker of damage in the central nervous system, the binding being predominantly to macrophages. Using photochemically induced focal cortical ischaemia as a lesion model in the rat, we have investigated the detection of secondary lesions using [3H]PK 11195 and ex vivo autoradiography. Secondary lesions in the thalamus became apparent during the second week post-lesioning, at a time when [3H]PK 11195 binding around the primary lesion was beginning to subside. Using Brain Browser software, the identity of the labelled thalamic nucleus was confirmed, objectively, as the ventrolateral nucleus, known to have reciprocal connections with the lesioned cortical area. As with the primary lesion, high densities of PTBBS correlated with infiltration of macrophages. Three-dimensional reconstruction of [3H]PK 11195 autoradiograph images showed binding along white matter tracts between the primary and secondary lesions. We conclude that radiolabelled PK 11195 given in vivo can be used in the visualisation of secondary lesions and their associated degenerating tracts.