A comparative study of the development of uptake hydrogenase and nitrogenase activities in cells of the cyanobacterium Anabaena variabilis was performed. The induction of heterocysts is followed by the induction of both in vivo hydrogen uptake and nitrogenase activities. Interestingly, a low but significant H2-uptake [2-7 μmoles of H2 . mg-1 (Chl a) . h-1] occurs in cultures with no heterocysts and with no nitrogenase activity. A slight stimulatory effect (30-40%) of H2 on in vivo H2-uptake was observed during the early stages of nitrogenase induction. However, exogenous H2 does not further stimulate the induction of in vivo hydrogen uptake observed during heterocyst differentiation. Similarly, organic carbon (fructose) did not influence the induction of either in vivo hydrogen uptake or nitrogenase activities. Exogenous fructose supports higher in vivo hydrogen uptake and nitrogenase activities when the cells enter late exponential phase of growth.