This report reviews the structural and functional characteristics of the ribosomal RNA genes (rDNA) of the ciliate protozoan, Tetrahymena. The study of rDNA was initiated some 10 years ago in order to establish a model system for rRNA gene expression in lower eukaryotes. The system proved very useful for studying several aspects of rRNA gene expression so that a considerable amount of information on rDNA structure and function has accumulated during the past years as a result of studies done in several laboratories. The initial finding that starved Tetrahymena cells, upon refeeding, preferentially replicated their rDNA for up to 90 min before bulk macronuclear DNA synthesis resumed, greatly facilitated the first studies. This observation permitted the rDNA to be labeled selectively with isotopes and in this way it was possible to isolate rDNA in pure form, to study replicative intermediates of the rDNA and to study rDNA containing chromatin in intact nuclei without resorting to chromatin fractionation. As a result of these studies it was found that the macronuclear rDNA of all Tetrahymena species consisted of a homogeneous population of extrachromosomal, linear molecules with a size of about 20 kilobase-pairs (kb) containing two identical transcription units for pre-rRNA arranged in a reverse repeat orientation (palindromic symmetry). The origins of replication were localized to the central non-transcribed spacer region and it was shown that this region has a chromatin structure which is different from that of the transcribed region. The primary DNA sequence is now known in many parts of the rDNA molecule, including the central part (containing the replication origins), the telomeric parts and the regions containing the sites of transcription initiation and termination. Transcription studies demonstrated the presence of an intervening sequence (intron) in the 26S rRNA coding region in some strains of Tetrahymena. Interestingly, the intron is transcribed and later removed from the primary transcript as a result of a rather unusual reaction which can take place in vitro in the absence of added protein factors. The finding of interbreeding strains of the intron+ and intron- rDNA genotype provided physical markers on the genes and have made possible a description of the inheritance and allelic assortment of the Tetrahymena rDNA. These studies proved that the free, palindromic rDNA molecules of the macronucleus arise from a chromosomally integrated, micronuclear rDNA copy as a result of a conjugational dependent amplification event, and that the intron is inherited as a neutral character during sexual and vegetative reproduction.(ABSTRACT TRUNCATED AT 400 WORDS)