The Vostok core provides the longest ice record of past climatic and environmental conditions, extending back to the interglacial Marine Isotope Stage (MIS) 11, around 400 kyr ago. The MIS 11 information obtained from different time series at Vostok, namely ice stable isotopes, CO2, CH4, Δ18O2, air content, is discussed in terms of extreme conditions of Antarctic temperature and atmospheric CO2, as well as ice sheet stability in the Vostok area. Comparison between ice and marine chronologies suggests that the Vostok MIS 11 ice may have recorded climatic conditions close to those of the optimum of this interglacial period. The phase relationship between atmospheric CH4 and the isotopic ratio of molecular atmospheric oxygen, as recorded in the Vostok core supports this conclusion. On the other hand, the Vostok ice deuterium-excess record gives support for a missing part of the warmest period of MIS 11. The LLN 2-D NH climate model is used to investigate how sensitive the simulated Northern Hemisphere MIS 11 de-glaciation is to the interplay between insolation and Vostok CO2 forcings. It is shown that the length of the simulated interglacial depends strongly on the phasing between these two climate forcings. Finally, we investigate the response of the Antarctic ice sheet to changing climate between MIS 11 and modern conditions through simulations performed with the LGGE 3-D ice sheet model. The results indicate that sea level stands during MIS 11 as high as 20m above present level cannot be accounted for, except by assuming that MIS 11 was very dry over Antarctica. © 2003 by the American Geophysical Union.