In the JT-60 tokamak with nearly perpendicular neutral hydrogen beam injection of up to ∼ 21 MW, some regimes of plasma species and beam directions have been observed in which the sawtooth periods are significantly extended. It is found that the use of helium as working gas together with co-injection of neutral beams can have an important influence on the occurrence of extended sawtooth periods. Beam induced stabilizing effects on sawteeth and internal kink modes are also discussed.