This paper describes eXAT, an experimental agent programming platform, based on the Erlang language, designed by the authors. The platform provides an "all-in-one" framework for the design, with a single tool, of agent intelligence, agent behaviour and agent communication. This is made possible in eXAT by means of the integration of an Erlang-based expert system engine, together with an execution environment for agent behaviours based on object-oriented finite-state machines. Such a system allows transition triggering on the basis of agent's mental state. Similarly, the provided agent communication facility, which is based on the exchange of ACL speech acts, can not only trigger behaviours but also influence agent's mental state according to FIPA-ACL semantics.