Changes in dielectric response features of lead lanthanum zirconate titanate (PLZT) ceramics by manganese addition are reported in the present work. Both the PLZT relaxor behaviour and the frequency dependence of the room temperature dielectric constant are shown to be suppressed by the addition of manganese. Similar results are obtained by applying a bias external field or by chemical reduction of PLZT samples, thus indicating the presence of an interrelationship between oxygen vacancies and the random ‘bias’ electric field. Consequently, the defects created by oxygen vacancies appear as the most important factor causing the disappearance of relaxor behaviour in Mn-doped PLZT ceramics. No modifications in the diffuse phase transition with the manganese content are found, which may be due to the persistence of compositional disorder. Additionally, it was verified that the spin glass model cannot be used to describe these observed effects.