Cyclic electron transport and NADH and/or NADPH (NAD(P)H)-oxidizing activities were investigated in Synechocystis sp. PCC6803 grown under various stressed conditions and in ndhB-less (M55) and ycf33-deletion mutants. Activity staining and inhibitor data suggested that the ferredoxin-quinone reductase (FQR) route is the main pathway in ycf33-deletion and high-light (300 microE m(-2) s(-1))-grown cells as well as in M55 cells. The FQR route was highly sensitive to HgCl(2), but not to diphenyleneiodonium (DPI). On the other hand, cells grown under low CO(2) (0.03%) or normal (100 microE m(-2) s(-1), 3% CO(2)) conditions were found perhaps to use the complex I-type NAD(P)H dehydrogenase route, which was found to be highly sensitive to DPI but not to HgCl(2). In high-salt (0.55 M NaCl)-grown cells, the amount of ferredoxin-NADP(+) oxidoreductase (FNR) increased, and the main cyclic electron flow was perhaps the FNR route. Both DPI and HgCl(2) were strong inhibitors of the FNR route.