We study the magnetic-field influence on the martensitic transformation temperatures and the accompanying anomaly in the resistivity by extensive measurements of the temperature dependencies of resistivity of several Ni-Mn-Ga and Ni-Fe-Ga alloys. A low-field minimum of the martensitic transformation temperature versus magnetic-field curves is observed for single and polycrystalline Ni-Mn-Ga alloys. This minimum is confirmed by magnetization loop measurements showing a magnetic anisotropy contribution to the ordinary Clausius-Clapeyron relationship which describes the martensitic transformation. This minimum did not appear for Ni-Fe-Ga polycrystalline alloys because in this case the difference in the magnetic anisotropy of relevant phases is small. We did not find any systematic behavior of the magnetoresistance related to the phase in which it is measured. In the vicinity of the martensitic transformation, the magnetoresistance exhibits a peaklike behavior which is explained in terms of the combined influence of the zero-field resistivity anomaly at the transition and the magnetic-field-induced shift of the transformation temperature.