We present a realizability interpretation of co-inductive types based on partial equivalence relations (per's). We extract from the per's interpreta- tion sound rules to type recursive definitions. These recursive definitions are needed to introduce «infinite» and «total» objects of co-inductive type such as an infinite stream or a non-terminating process. We show that the proposed type system enjoys the basic syntactic properties of subject reduction and strong normalization with respect to a confluent rewriting system first studied by Gimenez. We also compare the proposed type system with those studied by Coquand and Gimenez. In particular, we provide a semantic reconstruction of Gimenez's system which suggests a rule to type nested recursive definitions.