The Polymerase Chain Reaction (PCR) is one of the most widely used techniques in molecular biology (U.S. Pat. No. 4,683,202 to Mullis). In general, most thermocyclers which automate the PCR nucleic acid amplification process rely upon programmable heat blocks with a large thermal mass. Consequently, most of the time in an automated PCR cycle is spent non-productively in transition between denaturation, annealing, and elongation temperatures. Recently, much faster hot-air thermocyclers have been constructed which shorten these transition times, allowing 30 cycles of PCR in 10 to 30 minutes. While elegant in principle, the design of these systems is not optimal. Air is a relatively poor heat transfer medium; and the operation of a single heat/reaction chamber at atmospheric pressure is inherently slow. Much faster thermocyclers can be constructed using pressurized gas delivered to a thermostated reaction chamber by computer-controlled electronic valves. A novel process, high-speed gas phase PCR, is described. This process has been successfully automated using a novel thermocycing device, which has been successfully to amplify DNA from picogram to microgram amounts in ~1 to 5 minutes.