The natural cosmogenic radioisotope 36Cl has been measured in the Greenland Ice Core Project ice core from Summit, Greenland. Generally, a good correlation between 36Cl and Δ18O was found, even in glacial periods with strong Δ18O oscillations. The 36Cl flux is calculated assuming an exponential relationship between accumulation rate and Δ18O. A prominent excursion is detected between 35 and 40 kyr B.P., which coincides with a similar 10Be peak measured in the same samples. This sheds light on possible causes of this excursion. In the lowest part of the ice core the flux decreases smoothly. This can be interpreted as radioactive decay of 36Cl (T1/2=300 kyr). ¿ 1997 American Geophysical Union<