1. The diuretic drug ethacrynic acid (EA) is a potent reversible inhibitor of rat and human glutathione S-transferases (GST), with I50-values (microM) of 4.6-6.0, 0.3-1.9 and 3.3-4.8 for alpha, mu and pi-class, respectively. 2. The reversible inhibition by the glutathione conjugate of EA is even stronger for alpha and mu-class, with I50-values (microM) of 0.8-2.8 and < 0.1-1.2, respectively, while the I50 for the pi-class is 11. 3. Inhibition of rat and human pi-class GST also occurs by covalent binding of ethacrynic acid. 14C-ethacrynic acid, 0.8 nmol EA per nmol pi-class GST could be incorporated, resulting in 65-93% inhibition of the catalytic activity. 4. Owing to the chemical nature of the covalent binding (Michael addition), this reaction should be reversible. Indeed, full restoration of the catalytic activity of GST P1-1 inactivated by covalently-bound EA was reached in about 125 h by incubation with an excess of glutathione. 5. EA has been used to inhibit GST in biological systems. The reversible covalent binding may very well play a role in the observed inhibition of GST by EA in vivo.