The morphology and distribution of neurons and terminals that are immunoreactive to glutamic acid decarboxylase (GAD) were investigated in barrel cortex of the rat and mouse. The morphology of the GAD-immunoreactive neurons located in layer IV of the barrel field resembles that of the large, smooth stellate neurons described previously in Golgi studies. Most of the somas of GAD-positive neurons are located along the sides of the barrels. They constitute about 13 to 15% of the total neuronal population in layer IV. The spatial distribution of GAD-positive terminals in layer IV is similar to the distribution of GAD-positive somas. Very few GAD-positive neurons and terminals are found in the septal regions. This unique distribution of GAD immunoreactivity in the barrel cortex may serve as a model to study cortical inhibitory mechanisms.