This is a study of the form, density, and distribution of glutamic acid decarboxylase (GAD) immunoreactive neurons and puncta (axon terminals) in the adult rat medial geniculate complex. GAD-positive elements were stained by either the peroxidase-antiperoxidase or avidin-biotin procedures. Thalamic architectonic subdivisions were defined independently in Golgi, Nissl, plastic-embedded semi-thin, and fiber-stained preparations, and from investigations of medial geniculate connectivity. GAD-positive neurons represent only approximately 1% of medial geniculate neurons. They occur in the three major medial geniculate subdivisions (ventral, dorsal, and medial). There is variability between subdivisions in the form and number of such neurons, and among the puncta. In the ventral division, immunopositive somata may have sparsely branched dendrites as long as 300-400 microns and capped with varicose expansions or bouton-like sprays of appendages. These closely appose the somata or primary dendrites of other cells; the axons of these GAD-positive neurons are also immunostained. In the dorsal division there are fewer GAD-positive neurons and their structure is different. Their dendrites are rarely immunoreactive for more than 100-150 microns; nor can their immunostained axons be traced very far. In the medial division the number of GAD-positive neurons, considering the relatively small size of this division, was high. These neurons rarely have immunostained dendrites, and more than one type of neuron is immunoreactive. The average somatic diameter of GAD-positive neurons is about 60% of that of non-immunostained cells in semi-thin material; however, the range of somatic area and the dendritic variability of these neurons suggest that cells representing more than one population are immunopositive and include all but the largest neurons. The puncta also show regional differences. Small (0.5-2 microns in diameter), medium (2-3 microns), or large (greater than 3 microns) puncta occur. In the ventral division, the predominantly medium-sized puncta are about four times as numerous on a unit/area basis than in the dorsal division, where they are far smaller and more delicate; medial division puncta are as numerous as those in the ventral division, but are much larger and coarser, and may form perisomatic arrangements. Controls were devoid of specific immunostaining.(ABSTRACT TRUNCATED AT 400 WORDS)