Hydrogen production by autotrophic, vanadium-grown cells of Anabaena variabilis PK84, a cyanobacterial mutant impaired in the utilization of molecular hydrogen, has been studied under simulated outdoor conditions. The cyanobacterium was cultivated in an automated helical tubular photobioreactor (4.35 L) under air containing 2% CO2, with alternating 12-h light (36 degrees C) and 12-h dark (14 degrees to 30 degrees C) periods. A. variabilis steadily produced H-2 directly in the photobioreactor during continuous cultivation for 2.5 months. The maximum H-2 production by the continuously aerated culture under light of 332 mu E . s(-1) . m(-2) was 230 mL per 12-h light period per photobioreactor and was observed at a growth density corresponding to 3.6 to 4.6 mu g Chl a . mL(-1) (1.2 to 1.6 mg dry weight mL(-1)). Replacement of air with an argon atmosphere enhanced H-2 evolution by a factor of 2. This stimulatory effect was caused mainly by N-2 deprivation in the cell suspension. A short-term decrease of the CO2 concentration in the air suppressed H-2 evolution. Anoxygenic conditions over the dark periods had a negative effect on H-2 production. The peculiarity of hydrogen production and some physiological characteristics of A. variabilis PK84 during cultivation in the photobioreactor under a light-dark regime are investigated.