Particle control is a crucial point for the next step machines : particle injection and extraction systems must regulate the D-T fuel densities, exhaust helium ash, control the impurity core contamination and minimize the tritium wall inventory. Most of our knowledge on long-term fuel retention is from post mortem surface analysis of tiles, which will, however, not be possible in ITER. Thus the evaluation of gas balance data in present tokamaks is of high priority to establish the technique and database for ITER, in which gas balance will very likely be the dominant technique to assess the fuel retention. A series of identical discharges has been performed in JET for a full day, in order to study the particle retention behaviour as a function of the wall inventory and the global balance for a significant number of discharges associated to a high gas injection. The resulting particle balance over this day is presented in this paper.