Repeated air drying and rewetting of three soils followed by incubation at 20°C resulted in an increase in the rate of decomposition of a fraction of 14C labeled organic matter in the soils. The labeled organic matter originated from labeled glucose, cellulose and straw, respectively, metabolized in the soils during previous incubation periods ranging from 1.5 to 8 years. Air drying and rewetting every 30th day over an incubation period of 260–500 days caused an increase in the evolution of labeled CO2 ranging from 16 to 121 per cent as compared to controls kept moist continuously. The effect of the treatment was least in the soil which had been incubated with the labeled material for the longest time. Additions of unlabeled, decomposable organic material also increased the rate of decomposition of the labeled organic matter. The evolution of labeled CO2 during the 1st month of incubation after addition was in some cases 4–10 times larger than the evolution from the controls. During the continued incubation the evolution decreased almost to the level of the controls, indicating that the effect was related to the increased biological activity in the soils during decomposition of the added material. Three additions of organic material during the period of incubation resulted totally in an increase over the controls ranging from 36 to 146 per cent.