A thermal cycling method and device is disclosed. The device comprises a sample chamber whose temperature can be rapidly and accurately modulated over a range of temperatures needed to carry out a number of biological procedures, such as the DNA polymerase chain reaction. Biological samples are placed in containers each comprising a reservoir and a reaction portion, wherein the reaction portion has a small volume. The small volume reaction portion permits the rapid and accurate temperature modulation. With an optically transmissible reaction portion, DNA amplification may be monitored by fluorescence during PCR.