The in situ irradiation growth kinetics of two specimens of Zr-0.25 at.% Al doped with 0.06 at.% : 2 3 5 U: specimen RX (fully recrystallized) irradiated at 533 K in the CNEA-RA1 at a dose rate of 3.3 x 10 - 8 dpa s - 1 , and specimen CWSR (50% cold-worked and stress relieved), irradiated at 618 K in the CNEA-RA3 at a dose rate of 6.6 x 10 - 7 dpa s - 1 , are presented. The experimental method, based on the continuous deflection measurement and on the use of fissionable doped material to enhance the fast neutron damage, allows measurement of the growth phenomenon from the early stages of the irradiation. The transient or Stage I results of the irradiation growth behaviour obtained are in good agreement with results shown in the literature.