Abstract: The invention relates to a polymerase chain reaction (PCR) process and a thermal cycler. In the process, biological samples are held in a sample carrier having a plurality of sample spaces each having upper and lower ends and the samples are sequentially heated and cooled. The thermal cycler according to the invention comprises heat transfer means for automatic heating and cooling of the samples in the sample carrier, heatable closure means above the upper ends of the samples spaces for preventing condensation of sample vapor during the process, and adjusting means for controlling one process parameter, preferably the temperature of the heatable closure means, depending on at least one other of said process parameters. The invention helps to decrease the number of failed PCR experiments, in particular due to changes in sample volume.