RNA molecules are commonly produced in vitro by transcription, utilizing a DNA template, an RNA polymerase enzyme, and nucleoside triphosphate substrates (NTPs). In addition to the full-length RNA molecule coded for by the DNA template, significant amounts of shorter RNA molecules are produced. A simplified model of this complex transcription process is presented, with the shorter RNA molecules lumped into a single pool. The rate equations do not depend on the stoichiometry of the RNA molecule of interest, which facilitates application of the model to other RNA molecules. Optimal initial conditions for batch in vitro RNA transcription to produce a dodecamer RNA containing three different nucleotides have been predicted using the model. The predicted optimal values for equimolar NTPs are 10 to 15 mM initial concentration for each NTP and 50 to 60 mM for magnesium acetate, yielding a maximum final dodecamer concentration of 0.8 +/- 0.1 mM at the 90% confidence interval. Experimental data agree well with the model results. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 210-220, 1997.