Rhodospirillum rubrum was grown in a malate medium without bound nitrogen anaerobically under N2. 1. Resting cells of these cultures showed a gradual decrease in photoreducing activity (light-induced consumption of CO2 with H2). Small amounts of ammonium chloride immediately restored the initial activity. 2. A stoichiometric relationship was found between the amount of ammonium chloride added and the amount of gases which disappeared during the ensuing photoreduction: 8 H2 and 4 CO2 were taken up per mole NH3. 3. Addition of l-malate under photoreducing conditions (=atmosphere of 4% CO2 in H2) led to a substrate dependent gas evolution of H2 and CO2. Simultaneous addition of l-malate and various amounts of ammonium chloride gave at first only a comparatively small gas evolution which levelled off into a plateau. 4. The subsequent gas exchange following this first step depended upon the ratio of l-malate to NH4Cl. Titration of l-malate revealed that 2.2±0.3 moles of malate were consumed per mole of ammonium chloride. If there were more than 2.2 moles of malate present per mole of ammonium chloride, gas evolution was resumed after a characteristic time interval. If, on the other hand, there was less malate available, a strong gas uptake, photoreduction, was initiated. 5. In the presence of NH4Cl, malate was metabolized about 3 times faster than in its absence. 6. These observations are discussed in regard to the interrelationships between photosynthetic CO2 reduction, nitrogen fixation and protein metabolism of R. rubrum.