In type I (insulin-dependent) diabetes, destruction of pancreatic beta cells has been associated with the presence of circulating antibodies against glutamate decarboxylase (GAD), a GABA (gamma-aminobutyric acid) synthesizing enzyme which is located in the beta cells. We examined whether destruction of islet beta cells can lead to discharge of GAD in the extracellular medium, making it a potential autoantigen. Rat islet beta cells were first exposed for 1 hour to streptozotocin and then cultured for 4 to 24 hours before cellular and medium GAD activities were measured. After 24 hours culture, 70 percent of streptozotocin-treated beta cells were disintegrated whereas the number of control cells remained unchanged. Control cells exhibited a stable cellular GAD activity over the 24 hour period with no enzyme activity detectable in their culture medium. The cells recovered 24 hours after streptozotocin treatment exhibited 10-fold lower levels of GAD-activity and of GABA; their culture medium contained GAD, its enzymatic activity reaching peak values after 10 hours. The beta-cell enzymes glutamate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase were not detectable in the medium of control or streptozotocin-treated cells. Similar observations were made when beta cells had been exposed to cytotoxic concentrations of alloxan. It is concluded that damage to rat islet beta cells results in transient discharge of GAD in the extracellular medium making this enzyme a candidate extracellular marker for beta cell toxic processes and a potential autoantigen for immune reactivity.