A method in which a temperature gradient is generated across a "gradient" block, and an apparatus comprising a block across which a temperature gradient can be generated. By setting up such a gradient, multiple reaction mixtures held in wells on the gradient block can be simultaneously run at temperatures which differ only slightly, thereby permitting an optimum temperature for the reaction to be quickly identified. In a preferred embodiment the gradient block is integrated into a thermal cycler used for nucleic acid amplification reactions.