The use of oligonucleosomes of known composition and of simple efficient transcription systems has advanced the investigation of basic structure-function relationships in chromatin. This review article focuses on the structural and functional properties of physiologically-spaced nucleosomal arrays, using as a functional probe transcription by a bacteriophage RNA polymerase. This experimental approach has allowed the establishment of the following structure-function relationships. The absence of the histone H2A·H2B dimers from the core histone octamer, in addition to promoting initiation, allows the elongation of RNA chains as easily as in the corresponding naked DNA. Elimination of the tail domains of core histones is accompanied by partial release of the inhibition caused by core histone octamers on both transcription initiation and elongation. Incorporation of one molecule of the linker histone H5 into each nucleosome element does not affect transcript elongation either at the level of individual nucleosomes or when the presence of H5 induces stabilization of the 30-nm fiber. However, when an excess of H5 is bound inhibition of RNA elongation ensues.