Abstract: Structural operational semantics can be studied at the general level of distributive laws of syntax over behaviour. This yields speciﬁcation formats for well-behaved algebraic operations on final coalgebras, which are a domain for the behaviour of all systems of a given type functor. We introduce a format for speciﬁcation of algebraic operations that restrict to the rational ﬁxpoint of a functor, which captures the behaviour of finite systems. In other words, we show that rational behaviour is closed under operations speciﬁed in our format. As applications we consider operations on regular languages, regular processes and finite weighted transition systems.