If R is a ring with 1, we call a unital left R-module M Hopfian (co-Hopfian) in the category of left R-modules if any epic (monic) R-module endomorphism of M is an automorphism. In the case R is a commutative Noetherian ring, we use a result of Matlis to characterize those injective R-modules that are co-Hopfian, and to characterize those that are Hopfian when R is also reduced. We show that if R is a commutative Artinian principal ideal ring, then an R module M is Hopfian (co-Hopfian) if and only if M is finitely generated if and only if its injective envelope E(M ) is Hopfian (co-Hopfian) if and only if E(M ) is finitely generated. We note that the "finite uniserial type" problem poses an obstacle to establishing this result for an arbitrary Artinian principal ideal ring.