Purpose: The paper aims to present the concept of the functionality of metadata harvesting for regional digital libraries, based on the OAI-PMH protocol. This functionality is a part of regional digital libraries platform created in Poland. The platform was required to reach one of main objectives of the Polish PIONIER Programme--to enrich the content-based services in the Polish NREN. Design/methodology/approach: In order to create the regional digital libraries platform with metadata harvesting feature, digital library low-level services provided by the dLibra Digital Library Framework in a service-oriented architecture (SOA) are deployed. The approach for defining the set of services started with splitting the whole set of advanced digital library features into coherent functional groups, which covered functionality related to digital content, metadata, searching, etc. Then each functional group implicated the creation of a respective digital library service. Additionally, a couple of supporting services like "system" or "event services" were defined. Findings: The exploration of metadata and information about libraries' structure enables the creation of higher-level, content-based services like virtual collections or metadata searching, which are described in the paper. The approach for building distributed regional digital libraries in the National Research and Education Network is suitable for larger scale systems, like international digital libraries. This issue is very important for research networking, especially in the context of European Commission's announcement promoting digital access to Europe's heritage through the European Digital Library. Research limitations/implications: The implementation of the communication protocol between different instances of digital libraries is the basis for the development of content-based services. It presents examples of such services that are enabled through the PIONIER platform for distributed regional digital libraries, and that are provided for research and education users. The next step that will allow for the creation of even more sophisticated services, will offer the possibility to search through content gathered in PIONIER digital libraries. This will be achieved by combining the OAI-PMH protocol together with the MPEG-21 DIDL standard for the exchange of the digital content and content-related metadata. Another group of complementary services also covers information services provided by grid environments. Practical implications: The installation of regional digital libraries in the PIONIER network is currently providing access to more than 15,000 of digital publications. This paper presents a working implementation of the communication between digital libraries in the sense of the exploration of metadata and information about the library structure. This service is seen by us as one of the crucial mechanisms of the information infrastructure, which provides access to scientific and cultural content as well as common middleware services, which should be both distributed and platform-independent. Originality/value: The paper is providing the case study for those who are building the large-scale distributed digital library systems. The definition of services based on metadata harvesting provides an example of core digital library services, which are transforming digital libraries into digital infrastructure for social sciences. (Contains 3 figures and 5 notes.)