We report a study of the magnetoresistance (MR) of the metallic perovskite oxide as a function of the oxygen stoichiometry , magnetic field and temperature . We find a strong dependence of the nature of the MR on the oxygen stoichiometry. The MR at low temperatures changes from positive to negative as the sample becomes more oxygen deficient (i.e. increases). Some of the samples, which are more resistive, show resistivity minima at . We find that in these samples the MR is positive for and negative for . We conclude that in the absence of strong magnetic interaction, the negative MR in these oxides can arise from weak-localization effects.