A simple method for measuring changes in blood-brain barrier (BBB) permeability following neurotoxic lesions is described. In the brains of animals perfused transcardially with a trypan blue solution at the time of sacrifice, the presence of trypan blue staining correlated with changes in BBB function seen with more traditional markers, such as albumin staining. Thus, trypan blue appears to be useful as a marker for changes in BBB permeability. We have used this method to show increases in BBB permeability in striatal lesions induced by three different neurotoxins: chronic systemic injection of 3-nitropropionic acid (3-NP) and intrastriatal injection of either quinolinic or kainic acid. Trypan blue staining was seen in all three types of lesion, with both the neuropil and some neurones being stained. In the kainic acid lesioned animals, trypan blue also stained hippocampal and cortical neurones which are known to degenerate. Our findings suggest that trypan blue makes a more sensitive marker than albumin for both BBB integrity changes and degenerating neurones. Furthermore, this method has the advantages over others of being quick, economic and compatible with most subsequent histological and immunocytochemical staining.