The effects of partial substitution of Cu and Co for Ni in the intermetallic perovskite superconductor MgCNi 3 are reported. Calculation of the expected electronic density of states suggests that electron (Cu) and hole (Co) doping should have different effects. For MgCNi 3 - x Cu x , solubility of Cu is limited to approximately 3% (x=0.1). and T c decreases systematically from 7 to 6K. For MgCNi 3 - x Co x , solubility of Co is much more extensive, but bulk superconductivity disappears for Co doping of 1% (x=0.03). No signature of long range magnetic ordering is observed in the magnetic susceptibility of the Co doped material.