An apparatus for automatic performance of temperature cycles in a number of test tubes in which each test tube is closed by a closure and contains a predetermined volume of a liquid reaction mixture. The apparatus includes a holder, a computer-controlled automatic control system, and a device for cyclic alteration of the holder temperature. The holder has an arrangement of chambers for holding test tubes with each chamber being adapted to receive one test tube. The holder is constructed of a material of high thermal conductivity, and has an upper surface, a bottom surface and a cylindrical outer wall. Each chamber has an opening located in the upper surface of the holder. The device for cyclic alteration of the temperature of the holder is actuated by the automatic control system. By reducing the dimensions of the device and the power required to operate it, numerous advantages are realized. Such advantages are achieved through the arcuate arrangement of chambers in the holder, and the test tube closure which is piercable by a pipetting needle.