L-DEPRENYL is a selective inhibitor of monoamine oxidase (MAO) B, an enzyme predominantly localized in astrocytes. We have investigated the effect of treatment with L-deprenyl (0.25 mg kg-1 day-1) on glial fibrillary acidic protein (GFAP) immunoreactivity (IR) after lesioning the rat striatum with an injection cannula. No effect of drug treatment on GFAP IR was found in unlesioned striata. A significant increase (p < 0.01 vs saline treated rats) in GFAP IR was found in the tissue surrounding the lesion in striata of rats treated with L-deprenyl for 4 days after the lesion. When post-treated for 42 days, however, L-deprenyl no longer increased GFAP IR although reactive astrocytes were still present in the lesioned area. These results suggest that L-deprenyl can enhance the activation of astrocytes during a critical time-period following a striatal injury.