A b s t r a c t. In this paper we consider two rules of substitution for predicate symbols as introduced by Church, Introduction to mathematical logic, Princeton 1956, and by Pogorzelski, Prucnal, Structural completness of the first-order predicate calculus, Bulletin de l' Academie Polonaise des Sciences, serie des sci. math., ast. at phys., vol XXII, No.3(1974), 215-217. We show that the rules are not equivalent over a standard system of predicate logic.