The continuous charcoal record, interpreted with the aid of the results of combustion experiments, indicates that the mixing ratio of atmospheric oxygen has varied remarkably little over the past 350 Myr. We develop a dynamic feedback model of the coupled P, N, C, and O2 cycles and use perturbation analysis and a case study of the past 40 Myr to test various feedback mechanisms that have been proposed to stabilize atmospheric oxygen. These mechanisms involve alterations in nutrient driven productivity and the subsequent burial flux of organic carbon, which provides the main source of atmospheric oxygen. Suppression of the burial of phosphorus sorbed to iron minerals under anoxic conditions in ocean bottom waters tends to increase the ocean nutrient inventory and provide negative feedback against declining oxygen [Holland, 1994]. However, denitrification is enhanced by anoxia, tending to reduce the nutrient inventory and amplify declining oxygen [Lenton and Watson, this issue]. If organic phosphorus removal from the ocean is also suppressed under anoxic conditions, this improves oxygen regulation [Van Cappellen and Ingall, 1994], as does direct enhancement of organic carbon burial due to reduced oxygen concentration in bottom waters [Betts and Holland, 1991]. However, all of the ocean-based feedback mechanisms cease to operate under increases in oxygen that remove anoxia from the ocean. Fire frequency is extremely sensitive to increases in oxygen above 21% of the atmosphere, readily suppressing vegetation on the land surface. This should transfer phosphorus from the land to the ocean, causing less carbon to be buried per unit of phosphorus and providing a weak negative feedback on oxygen [Kump, 1988]. However, a new proposal that increases in oxygen suppress the biological amplification of rock weathering and hence the input of phosphorus to the Earth system provides the most effective oxygen regulation of all the mechanisms considered. A range of proxies suggests that the input of available phosphorus to the ocean may have been significantly reduced 40 Myr ago, suppressing new production and organic carbon burial in the model. With only ocean-based feedback, the atmospheric oxygen reservoir is predicted to have shrunk from ∼26% of the atmosphere 40 Myr ago. However, when land plant mediated negative feedback on phosphorus weathering is added, oxygen is regulated within 19–21% of the atmosphere throughout the past 40 Myr, in a manner more consistent with paleorecords.