Detailed measurements of Δ18O of atmospheric oxygen performed on air trapped in the Vostok ice cores (Antarctica) are used to extend the record of the Dole effect over two climatic cycles (back to 240 kyr B.P.). Except for glacial terminations I and II and for an unexpected minimum occurring around 175 kyr, the Dole effect shows small variations (ΔDole within ¿0.5?). These small variations, however, show a well-marked 23 kyr precessional periodicity, thus confirming the results obtained by Bender et al. <1994a> for the first climatic cycle. To explain the minimum value reached around 175 kyr, we invoke the possibility of a peak in the oceanic productivity linked to climatic events induced at low latitudes under glacial conditions. ¿ 1999 American Geophysical Union