Hydrogen production by Anabaena variabilis ATCC 29413 and of its mutant PK84, grown in batch cultures, was studied in a photobioreactor. The highest volumetric H-2 production rates of native and mutant strains were found in cultures grown at gradually increased irradiation. The native strain evolved H-2 only under an argon atmosphere with the actual rate as high as the potential rate (measured in small vials under optimal conditions). In this case 61% of oxygenic photosynthesis was used for H-2 production. In contrast the mutant PK84 produced H-2 during growth under CO2-enriched air. Under these conditions at the maximum rate of H-2 production (10 mt h(-1) L-1), 13% of oxygenic photosynthesis was used for H-2 production and the actual H-2 production was only 33% of the potential. Under an atmosphere of 98% argon + 2% CO2 actual H-2 production by mutant PK84 was 85% of the potential rate and 66% of oxygenic photosynthesis was used for H-2 production. Hydrogen production under argon + CO2 by the mutant was strictly light-dependent with saturation at about 300 mu E m(-2) s(-1) However, the rate of photosynthesis was not saturated at this irradiation. At limiting light intensities (below 250 mu E m(-2) s(-1)) 33-58% of photosynthesis was used for H-2 production. Hydrogen evolution by PK84 under air + 2% CO2 was also stimulated by light; but was not saturated at 332 mu E m(-2) s(-1) and did not cease completely in darkness. The rate of oxygen photoevolution was also not saturated. A mechanism for increasing cyanobacterial hydrogen production is proposed.