The total reducing activity (TRA) of cells was used to estimate the physiological activity of Corynebacterium glutamicum under conditions of L-lysine synthesis. This was estimated as the rate of reduction of 2,3,5- triphenyltetrazolium chloride by intact cells. TRA of cells was linearly correlated with the intracellular concentrations of RNA and the bacterial growth rate. It was concluded that this activity reflected the rate of energy generation in cells. A decrease in TRA of growing cells was related to an increase in bacterial lysine synthesis activity. Alteration in metabolic pathway functioning and an increase in the intracellular concentrations of lysine precursors favoured an increase in the rate of lysine synthesis by bacterial cells when cellular TRA was decreasing.