A brief review of the carbon and nitrogen metabolisms in blue-green algae (cyanobacteria) had is been presented on the basis of cell biology. Semi-steady state continuous culture of Anabaena variabilis ATCC 29413 (a heterocystous blue-green alga) had been was set up and was applied to the physiological research using two types of apparatus, i.e., a BioFlo chemostat and a small whole-glass fermentor. Under a light-dark (12:12) regime the culture reached to the semi-steady state in 5-6 days. Physiological activities of the cells maintained on a stable level, growth parameters of the culture met the relationship expressed in the equation X = X_oe~(μ-D)t. Specific growth rate and the change rates of cell components followed the light-dark cycle and were different from each other. During the light period biomass increased due to CO_2-assimilation and nitrogen-fixation, due to the continuous dilution and respiration it decreased approximately at the same rate in the dark. The increase of biomass in the light was mainly due to the synthesis and accumulation of glycogen. Glycogen accumulated up to 20% of biomass at the end of the light period, then degraded down to 1-2% of dry weight in the dark. The synthesis rate of glycogen during light period was not uniform being highest at the onset of illumination (2.6 μg.μg~(-1) chl.h~(-1)). The culture was CO_2 - limited when supplied with normal air (0.03%. CO_2). Higher CO_2 (1%) supply caused that the biomass increased to 1.6-1.8 times and the concentration of glycogen in culture also increased correspondly. Synthesis rate of glycogen increased rapidly in the transitional phase from low CO_2 to high CO_2 supply, it was 6.1 μg.μg~(-1) chl.h~(-1) and the glycogen per dry weight was 56% at the beginning of the phase. Afterwards it decreased gradually day and day to a new semi-steady state. According to the data the affinity of inorganic carbon to the cell had been discussed and a possible model for the CO_ and carbonate transport system of the cell was proposed. Adding ammonium to the culture stimulated the increase of biomass but the accumulation of glycogen decreased. Simultaneously supplying NH_4~+ and enriched CO_2 to the culture led to a enhancement of biomass but not the accumulation of glycogen in cells. Nitrogenase activity was low before the onset of light period, then it increased rapidly in the following hours. Maxium of light-induced nitrogenase activity was reached 4 to 6 hours after light on followed by a decrease due to enzyme turnover. Nitrogenase synthesis in the heterocystous blue-green alga, A. variabilis, only occurred in the first hours of light period. Supplying ammonium and chloramphenicol to the culture at the onset of light, the nitrogen activity was inhibited strongly. Throughout the diurnal cycle nitrogenase activity determined in dark assay was one fifth of light-stimulated activity. No correlation was found of dark activity with the glycogen content of whole filaments. Glycogen derived from photosynthesis did not determine the nitrogenase activity in the dark. Also nitrogenaso activity was calculated from the increase of total nitrogen measured. The number of electrons for ammonium formation indicated that no free hydrogen was produced. More than 90% of nitrogen-fixation occurred during the light period, nitrogen-assimilation of A. variabilis was essentially bound to the light period. No significant change in the nitrogen content of the biomass was observed during the light-dark cycle, but the nitrogen content of extracted proteins increased in the light and decreased in the dark indicating that nitrogen assimilation in the light was reserved transiently in nitrogen-rich proteins. The semi-steady state of continuous culture of a heterocystous blue-green alga was successfully set up for the first time in the University Constance, F. R. Germany (Prof. Boger's statement, 1987). It is a new method for the study of algal physiology in our country as well. The results of glycogen synthesis, accumulation, degradation and the nitrogenase synthesis, activity as well as the nitrogen content of proteins related with nitrogen assimilation under light-dark cycle were reported here for the first time. Interest from these results would be expected in algal physiology, eco-physiology and algal biotechnology.