CHANGES in ocean chemistry and circulation have been invoked to explain the lower atmospheric CO2 concentrations of glacial periods observed in ice-core records’. The processes that modulate these concentrations are not well understood, but an increase in the nutrient inventory of the ocean is one mechanism that could lower atmospheric CO2 levels by enhancing oceanic biological productivity and CO2 storage(1-3). The oceanic concentrations of one such nutrient, nitrate, may be regulated by changes in the rate at which it is degraded by bacteria (denitrification) in oxygen-deficient subsurface waters. Denitrification constitutes a significant global sink for oceanic nitrate(4) and the eastern tropical North Pacific Ocean is particularly important in this respect as it accounts for at least a third of global oceanic fixed-nitrogen removal by water-column denitrification(4,5). Here we present N-15/N-14 records from marine sediment cores, which show that water-column denitrification in the eastern tropical North Pacific Ocean was greatly diminished during glacial periods. We suggest that, because nitrate limits biological productivity in much of the modern ocean, a consequent increase in the oceanic nitrate inventory during glacial periods could have contributed to the observed decrease in atmospheric CO2 concentration.