Experimental and therapeutic use of islet cell preparations could benefit from assays that measure variations in the mass of living beta cells. Since processes of cell death can be followed by depletion and/or discharge of cell specific substances, we examined whether in vitro conditions of beta cell death resulted in changes in tissue and medium content of insulin and of GABA, two beta cell specific compounds with different cellular localization and turnover. Exposure of rat purified beta cells to streptozotocin (5mM-120 min) or to the NO-donor GEA (50µM-120 min) caused 80 percent necrosis within 24 h; at end of this period, cellular insulin content was not significantly decreased but cellular GABA content was reduced by 70 percent; when cultured at basal glucose (6mM), the toxin-exposed cells did not discharge less insulin but released 80 percent less GABA in the period 8 to 24h. As in rat beta cell purification, GABA co-migrated with insulin during human islet cell isolation. Twenty-four hours after GEA (500µM-120min), human islet cell preparations exhibited 90 percent dead cells and a 45 and 90 percent reduction in, respectively, tissue insulin and GABA content; in the period 9 to 24 h, insulin discharge in the medium was not reduced but GABA release was decreased by 90 percent. When rat beta cells were cultured for 24h with non-toxic IL-1 concentrations that suppressed glucose-induced insulin release, cellular GABA content was not decreased and GABA release increased by 90 percent in the period 8 to 24 h. These data indicate that a reduction in cellular and medium GABA levels is more sensitive than insulin as marker for the presence of dead beta cells in isolated preparations. Pancreatic GABA content also rapidly decreased following streptozotocin-injection and remained unaffected by 12 hours of hyperglycemia. At further variance with insulin, GABA release from living beta cells is little dependent on its cellular content but increases with IL-1-induced alterations in beta cell phenotype.