The effects of temperature on the aging process have been investigated in approximately 3500 imagoes of male Drosophila melanogaster (Oregon R), with focus on the following parameters: mortality, O2 utilization, vitality (as expressed by negative geotaxis and mating) and fine structural alterations in the abdominal organs and brain. The data on mortality kinetics of flies maintained continuously at 18 degrees, 21 degrees, 27 degrees or 30 degrees C or exposed in succession to 21 degrees and 27 degrees C or vice versa support the concept that life span is temperature dependent. Moreover, these data, together with the increased O2 utilization and accelerated loss of vitality at 21 degrees C as compared with 18 degrees C, suggest that, in agreement with the rate-of-living theory proposed by Alpatov and Pearl in 1929, the flies are living faster at the higher temperature. Fine structural aging changes, like ribosomal loss in the Malpighian tubules and lipofuscin-ceroid accumulation in the midgut cells, seem to be more intense at 27 degrees and at 29 degrees C than at 21 degrees C. Also, the low vitality exhibited through their lives by flies kept at previous exposure through many generations) to 21 degrees C. Flies maintained at 29 degrees C showed a striking degeneration of the brain with an almost complete loss of the cytoplasmic organelles of the nerve cells. This electron microscopic finding lends support to the view that nervous tissue injury (perhaps induced by thermal denaturation of membrane lipoproteins) may play a crucial role in life shortening induced by relatively high temperatures.