Oxygen deprivation is a major cause of cellular damage and death. Here we demonstrate that Caenorhabditis elegans embryos, which can survive both in anoxia (<0.001 kPa O 2) by entering into suspended animation and in mild hypoxia (0.25-1 kPa O 2) through a hypoxia-inducible factor 1-mediated response, cannot survive in intermediate concentrations of oxygen, between 0.01 and 0.1 kPa O 2 . Moreover, we show that carbon monoxide can protect C. elegans embryos against hypoxic damage in this sensitive range. Carbon monoxide can also rescue the hypoxia-sensitive mutant hif-1(ia04) from lethality in hypoxia. This work defines the oxygen tensions over which hypoxic damage occurs in C. elegans embryos and demonstrates that carbon monoxide can prevent this damage by inducing suspended animation.O xygen metabolism is a fundamental requirement for life in aerobic metazoans. Aerobic respiration accounts for the vast majority of energy production in most animals and also serves to maintain the redox potential necessary to carry out important cellular reactions. In hypoxia, decreased oxygen availability results in inefficient transfer of electrons to molecular oxygen in the final step of the electron transport chain. This inefficiency results in both a decrease in aerobic energy production and an increase in the production of damaging free radicals, mainly caused by the premature release of electrons at complex III and the formation of O 2 Ϫ by cytochrome oxidase (1). Limited energy supplies and free radical damage can interfere with essential cellular processes, such as protein synthesis and maintenance of membrane potentials (2), and will ultimately lead to cell death.Hypoxia is a common natural stress, and several well conserved responses exist that facilitate cellular adaptation to hypoxic environments. To compensate for the decrease in the capacity for aerobic energy production in hypoxia, the cell must either increase anaerobic energy production or decrease energy demand (2). Examples of both of these responses are common in metazoans, and the particular response used depends, in general, on the amount of oxygen available to the cell.In mild hypoxia, oxidative phosphorylation is still partially active, so some aerobic energy production is possible. The cellular response to this situation, which is mediated in part by hypoxia-inducible transcription factor 1 (HIF-1), is to supplement the reduced aerobic energy production by up-regulating genes involved in anaerobic energy production, such as glycolytic enzymes and glucose transporters (3,4). This response also promotes the up-regulation of antioxidants such as catalase and superoxide dismutase, which guard against free radical-induced damage. As a result, the cell is able to maintain near normoxic levels of activity in mild hypoxia.In an extreme form of hypoxia, which we call anoxia and define here as Ͻ0.001 kPa of O 2 , oxidative phosphorylation ceases and thus the capacity to generate energy is drastically reduced. To survive in this environment, the cell must decrease ...