As an inhibitor of ferredoxin-NADP reductase (FNR), heparin strongly inhibited the Fd-mediated photoreduction of NADP in spinach thylakoids at very low concentration (micromole), and also inhibited the delayed light emission induced by Fd/NADP. Under anaerobic conditions, when DCMU was present to block electron transfer from photosystem II, the Fd-mediated cyclic electron transport around photosystem I as indicated by the quenching of 9-aminoacridine can be measured. Heparin could strongly inhibit this cycle, but had no significant effect on the PMS-mediated cyclic electron transport around photosystem I. Heparin did not inhibit the electron transport from phototsystem I to ferredoxin, which was demonstrated by measuring the photoreduction of ferredoxin. The above results indicate that heparin inhibits ferredoxin-mediated cyclic electron transport by inhibiting the activity of FNR, and support the hypothesis of the involvement of FNR in cyclic electron transport.