The transcription patterns of 64 linear double stranded DNA templates obtained with T7 RNA polymerase were investigated. These templates consisted of 17 nucleotide-long sequences under the control of the minimal bacteriophage T7 promoter and represented all possible combinations of nucleotides at positions +8, +10 and +11. Two clearly distinct types of template were identified, which produced the range of transcription patterns observed: (a) those that yielded 17-nucleotide-long RNA as the only detectable run-off product (only 15% of the total), and (b) templates that in addition to the expected full-length RNA, produced other products longer than 17 nucleotides. Self-complementarity analysis of the expected run-off transcripts showed that those obtained from the first type of template were able to form stable intermolecular duplexes with non-base-paired 3'-ends. However, the second type of template yielded RNAs able to generate energetically favorable intermolecular duplexes with 3'-end complementarity, therefore yielding an RNA-primed RNA-template. The gel-purified 17-nucleotide-long RNAs transcribed from the latter yielded longer products when incubated under in vitro transcription conditions in the absence of a DNA template. No extension was observed when assaying the 17-nucleotide RNA products resulting from the first type of template. We observed that just a single nucleotide change within the DNA template could convert the RNA product from an RNA-primed template into a nonextendible dimer thus leading to a drastic switch of the 17-nucleotide product yield from less than 10% to 100%. Further, two type B DNA templates were extended by two nucleotides at the 3'-end, to produce RNA transcripts theoretically unable to form 3'-end base-paired duplexes. The full-length products of these modified DNA templates were found to be nonextendible by T7 RNA polymerase under the standard in vitro transcription conditions.