Inactive NADP-malate dehydrogenase (disulfide form) from chloroplasts of Zea mays is activated by reduced thioredoxin while the active enzyme (dithiol form) is inactivated by incubation with oxidized thioredoxin. This reductive activation of NADP-malate dehydrogenase is inhibited by over 95% in the presence of NADP and the Kd for this interaction of NADP with the inactive enzyme is about 3 microM. Other substrates of the enzyme (malate, oxaloacetate, or NADPH) do not effect the rate of enzyme activation but NADPH can reverse the inhibitory effect of NADP. It appears that NADPH (Kd = 250 microM) and NADP (Kd = 3 microM) compete for the same site, presumably the coenzyme-binding site at the active centre. Apparently the enzyme . NADP binary complex cannot be reduced by thioredoxin whereas the enzyme . NADPH complex is reduced at the same rate as is the free enzyme. Similarly the oxidative inactivation of reduced NADP-malate dehydrogenase is inhibited by up to 85% by NADP and NADPH completely reverses this inhibition. The Kd values of the active-reduced enzyme for NADP and NADPH were both estimated to be 30 microM. From these data a model was constructed which predicts how changing NADPH/NADP levels in the chloroplast might change the steady-state level of NADP-malate dehydrogenase activity. The model indicates that at any fixed ratio of reduced to oxidized thioredoxin high proportions of active NADP-malate dehydrogenase and, hence, high rates of oxaloacetate reduction, can only occur with very high NADPH/NADP ratios.