- Modification of the magnetic force by a developed small-scale magnetohydrodynamic (MHD) turbulence can result in the sign reversal of the efictiue magnetic pressure. It is due to negative contribution of the MHD turbulence, to the large-scalem agnetic force. It can significantly lower the elasticity of the large-scale magnetic field [ Sov. Phys. JETP 70, 878 ( 1990)]. This effect excites instabilities of the large-scale magnetic field due to the energy transfer from the turbulent pulsations to the latter. The nonturbulent stability criteria are modified due to the effectiue negative magnetic pressure. These instabilities may provide a mechanism of the large-scale magnetic ropes formation in the solar convective zone and spiral galaxies. In addition, the instabilities can excite the short-period solar oscillations.