Matrix metalloproteinases (MMPs) are a group of zinc endopeptidases that break down the extracellular matrix. MMP-2 and MMP-9 are particularly abundant in the brain and are essential for synaptic plasticity. When studying synaptic plasticity, it is crucial to measure their activity in both normal and pathological conditions. Although gel and in situ zymography are commonly used to assess MMP proteolytic activity, these methods are typically applied to samples and tissues ex vivo and do not allow the quantification of MMP function during a behavioral experiment. Consequently, the in vivo zymography represents the ideal activity assay to study the MMP function. Moreover, we recently developed a method to quantify MMP activity in specific cell types in rat brains. This chapter details the in vivo zymography approach used to accurately quantify the gelatinolytic activity of MMP-2 and MMP-9 in rat brains in both non-cell-specific and cell-specific manners.