Helium-3 data of the upper water column (≤ 800 m) at stations in the northwestern Weddell Sea and in the Bransfield Strait are presented and discussed in relation to the hydrography. From our observations a 3He balance for the winter mixed layer is proposed, which allows us to estimate the rate of entrainment of Warm Deep Water into the Winter Water to be between 15 and 35 m y-1. The vertical eddy diffusivity within the pycnocline above the Warm Deep Water core is found to be approximately 5 × 10-5 m2 s-1. Beyond our use of 3He data, the difference to the oxygen balance approach of Gordon et al. (1984, Journal of Geophysical Research, 89, 637-640) is that they use winter data whereas we base our estimate on summer observations. Of the assumptions required to obtain the WDW entrainment estimate, that of quasi-stationarity of the gas transfer across the pycnocline appears to be the most critical one, but other assumptions appear to require further study as well. © 1987.