To investigate the release of glutathione (GSH) from brain cells, cultures enriched for astroglial cells, neurons, oligodendroglial cells, and microglial cells derived from rat brain were studied. During incubation of astroglial cultures, GSH accumulated in the medium with a rate of 3.1 ± 0.6 nmol × h−1 × mg protein−1. In contrast, only marginal amounts of extracellular GSH were detectable in the media of the other brain cell cultures investigated. The mechanism of GSH release from astroglial cells, as yet, has not been reported. Multidrug resistance protein 1 (Mrp1), a transport protein known to mediate cellular export of glutathione disulfide and glutathione conjugates, is expressed in astroglial cultures. Inhibitors of Mrp1 were used to test for a function of this transporter in mediating GSH release from astroglial cells. The presence of the competitive Mrp1 inhibitor MK571 at a concentration of 50 μM inhibited the rate of GSH release by 63%. In contrast, the low concentration of 1 μM of MK571 increased the rate of GSH release by 83%. This bimodal concentration-dependent effect of MK571 is in accord with literature data for the effects of Mrp1 substrates on GSH release from cells. In addition, the presence of cyclosporin A (10 μM) reduced the GSH release rate significantly and completely blocked the stimulating effect of 1 μM MK571 on the release of GSH from astroglial cells. In conclusion, the data presented are a strong indication that Mrp1 participates in the release of GSH from astroglial cells. © 2002 Wiley-Liss, Inc.