Abstract: A miniaturized temperature-zone flow reactor, used for thermally controlled biochemical or molecular-biological processes, especially polymerase chain reaction (PCR) enables more efficient reactions by providing at least one closed flow path which is divided into three partial paths (A1 . . . An; B1 . . . Bn and BB1 . . . BBn?1; C1 . . . Cn) with the reactor having three substrate chips (A; B; C) which are made of a material having a high heat conductivity, and which have defined channel sections that are spaced apart relative to each other, and are connected by a connecting chip (V) made of a poor heat-conductive material. The substrate chips (A; B; C) are maintained at different temperatures by various means, including the use of controlling heating elements in contact with the chips.