We determine the parameters of the Free Core Nutation (FCN) from the resonance induced in the forced nutations by a least-squares analysis of VLBI observations. By applying the procedure to consecutive observation series, we deduce the time evolution of the FCN period, and show that it varies only between 431 and 434 sidereal days. These bounds are within the limits of uncertainty arising from noise in the data. We therefore find no evidence for time variation of the FCN period. ¿ 1999 American Geophysical Union