Diaphorase and transhydrogenase activities (system NADPH; dichlorophenolindophenol and NADPH;NAD, respectively) of ferredoxin-NADP reductase are increased by ferredoxin. Reduced thionicotinamide-NADP (TN-NADPH) only slightly inhibits these activities with no ferredoxin present in the assay, but the activity increments in the presence of ferredoxin are strongly decreased. Photosynthetic pyridine nucleotide reduction is also inhibited by the reduced analog, the extent of inhibition being approx. in the same order with all three activities. The ferredoxin stimulated activities therefore appear to be due to the same interaction between reductase and ferredoxin in all three cases. The inhibition by TN-NADPH resembles that with Na-pyrophosphate (or NaCl), although in contrast to salt inhibition it is a.) not alleviated by higher ferredoxin concentrations, and b.) it still allows reductase/ferredoxin binding. The binding of the reduced pyridine nucleotide analog to the reductase seems to interfere rather specifically with the stimulation which ferredoxin can exert on the enzymic activities of the reductase.