The temperature dependence of the magnetic susceptibility between 100 and 850 K was measured for Mn 1 - x Ni x and (Mn 1 - x Ni x ) 4 N solid solutions. The investigated samples were checked by metallographic microscopy and all were found as solid solutions. For Mn 1 - x Ni x solid solutions the magnetic susceptibility increases linearly with temperature increase, obeying the relation χ = aT + χ 0 which suggests a longitudinal spin fluctuation mechanism. The samples were heat treated in a nitrogen atmosphere at 1173 K during 1 h. After the heat treatment, the samples exhibited itinerant ferromagnetic behaviour associated with the formation of the solid solution between Mn 4 N intermetallic compound and nickel, resulting in the system (Mn 1 - x Ni x ) 4 N.