The effect of a uniform magnetic field on the heat transfer of liquid metal forced flows in straight rectangular channels subjected to a uniform heat flux on one of the walls parallel to the field is experimentally studied. The experiments covered the domain 3.8 ⩾ U ⩾ 18 cm/s; 0 ⩾ B ⩾ 0.9 T; 0 ⩾ M ⩾ 300 and 4 × 103 ⩾ Re ⩾ 2 × 104. It is found that the application of the magnetic field improves the heat transfer in channels made of both non-conducting walls and of conducting walls. Whereas in the former case this improvement results from the creation of enhanced anisotropic turbulence across he entire channel, in the conducting channel the improvement results from an increase in the velocity near the heated wall as well as enhancement of turbulence in the vicinity of this wall.