Photo-fermentative production of hydrogen (H2) by purple bacterium Rhodobacter sphaeroides MDC6521 from Armenian mineral springs and its regulation by external reducers and oxidizers have been investigated. Reducers such as dl-dithiothreitol (DTT) and dithionite suppress bacterial growth but enhance H2 yield of R. sphaeroides, whereas oxidizer ferricyanide inhibits both processes. The effect of DTT on DCCD-inhibited FoF1-ATPase activity of R. sphaeroides membrane vesicles has been analyzed too. DTT increases the DCCD-inhibited ATPase activity. Thus, more negative values of Eh by addition of DTT might regulate FoF1-ATPase activity. The participation of the ATPase in redox sensing under photo-fermentative H2 production is suggested. This enzyme might be a target, having a significant role in these processes of purple bacteria.