A kinetic effect is evidenced in lead zinc niobate (PZN). We measured the time dependencies of the polarisation P(t) of a single crystal. At a given temperature and electric field, P(t) is described by the expression P(t) = P∞(T) f(t/τ(E,T)) where τ(E,T) is the incubation time above which the sample is completely polarised. The τ(E,T) was fitted using the exponential law exp[(W+kδT/E)/kT] where the activation energy W is equal to 0.91±0.36 eV and δT is equal to 2.2×108 K V/m in the 300-360 K temperature range. Taking into account this kinetic effect the electric-field-temperature diagram and the existence of an electric field threshold were discussed.