Relaxation of the isothermoremanent magnetization M(IR) has been studed in ceramic Y-Ba-Cu-O. The value of the effective pinning energy U(eff) has been determined at different temperatures according the relationship dlnM(IR)/dlnt = - kT/U(eff). It was shown that the dependence of U(eff) on the magnetizing field H(o) can be attributed to the evolution of the trapped flux distribution (as H(o) is varied) without involving any assumptions about the dependence of the pinning barrier on the field.