[en] Crack growth rates (CGR's) were determined under sustained and cyclic loads using 17 mm compact tension and cantilever beam specimens taken from Zr-2.5Nb tubes with 6 to 100 ppm H. Under sustained loads, the CGR of the Zr-2.5Nb tubes increased with increasing supersaturation of hydrogen, ΔC and leveled off above 20 to 25 ppm H of the ΔC. Under cyclic loads with 1 cycle/min, the CGR at 250 deg. C decreased with decreasing R. In contrast, striation spacing, corresponding to the critical hydride length, decreased with decreasing R, indicating easier cracking of the hydrides under cyclic loads. It is demonstrated that the governing factor for the CGR under the decreased ΔC and cyclic loads is the hydride nucleation rate which becomes slower than the rates of growth and cracking of hydrides. (authors)