With the proliferation of mobiles devices equipped with communication interfaces such as IEEE 802.11 or IEEE 802.15, the mobile ad hoc networks have been recently arousing an increasing interest, and are considered from now as relevant supports for the ubiquitous and pervasive computing. The service provision in such networks arises as a critical issue for the success of these new computing paradigms. In this paper, we present both an approach and a middleware platform for the discovery and the invocation of services in disconnected, partially connected or intermittently connected mobile ad hoc networks. This approach relies on a content-based management of messages, on a flexible service invocation and on context-awareness.