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The Siberian permafrost is an extreme, yet stable environment due to its continuously frozen state. Microbes maintain membrane potential and respiratory activity at average temperatures of −10 to −12°C that concentrate solutes to an a w = 0.90 (5 osm), The isolation of viable Psychrobacter arcticus sp. 273-4 and Exiguobacterium sibiricum sp. 255-15 from ancient permafrost suggests that these bacteria have maintained some level of metabolic activity for thousands of years. Permafrost water activity was simulated using ½ TSB + 2.79 m NaCl (5 osm) at and cells were held at 22 and 4°C. Many cells reduced cyano-tetrazolium chloride (CTC) indicating functioning electron transport systems. Increased membrane permeability was not responsible for this lack of electron transport, as more cells were determined to be intact by LIVE/DEAD staining than were reducing CTC. Low rates of aerobic respiration were determined by the slope of the reduced resazurin line for P. arcticus, and E. sibiricum. Tritiated leucine was incorporated into new proteins at rates indicating basal level metabolism. The continued membrane potential, electron transport and aerobic respiration, coupled with incorporation of radio-labeled leucine into cell material when incubated in high osmolarity media, show that some of the population is metabolically active under simulated in situ conditions. [ABSTRACT FROM AUTHOR]