L1210 cells treated with L-buthionine-(S/R)-sulfoximine (BSO) had glutathione (GSH) and non-protein thiol levels only 15% that of control. These GSH-depleted cells grew as well as the control L1210 cells and there was no decrease in ribonucleotide reductase activity in situ as measured by the conversion of [14C]cytidine to deoxytidine nucleotides and incorporation into DNA. Further, when these BSO-stressed cells were treated with hydroxyurea or IMPY, there was no potentiation of the inhibition caused by hydroxyurea or IMPY alone. These data indicate that the glutathione/glutaredoxin system of ribonucleotide reductase is not the sole carrier of reducing equivalents from NADPH for the reduction of the 2'-position of the corresponding ribonucleoside 5'-diphosphate; and that glutathione is not critical in regenerating the tyrosyl free-radical on the M2 subunit which is destroyed by the hydroxyurea or 2,3-dihydro-1H-pyrazolo-[2,3-alpha]imidazole (IMPY) treatment.