In three-week-old pea plants (Pisum sativum L., cv. Little Marvel) grown in the light, total glutathione levels were highest in apex and expanding leaves (1.5 μmol·(g FW)(-1)), and lower (0.4-0.6 μmol·(g FW)(-1)) in older leaves and roots. In the light period, levels in expanded leaves increased by about 40%, compared with dark values, with lesser increases in roots and apex. In illuminated plants the proportion in the reduced form was 86-88% in leaves and 80% in roots, and these values fell during the dark period (to 82% and 69%, respectively). Reduced glutathione makes up 65-70% of the low-molecular-weight thiol in leaves, but over 95% in roots. Chloroplasts contained about 10% of the leaf glutathione, at a concentration of 1-2 mM; total glutathione in the chloroplasts, and the proportion of oxidised form (GSSG) increased in light, while NADP(+)/NADPH ratios decreased, indicating both synthesis and active oxidation of glutathione in light. Chloroplasts contained 52% (young leaf) to 75% (mature leaf) of the GSSG-reductase (EC 188.8.131.52) activity in the leaves. In roots, over 30% of the GSSG reductase was in the plastid fraction. Very little GSSG-reductase activity was associated with mitochondria in leaf or root.