In 1995 at the integral test facility ISB-VVER in Elektrogorsk near Moscow, natural circulation experiments were performed that were scientifically supported by the Forschungszentrum Rossendorf. These experiments were the first of this kind at a test facility that models VVER-1000 thermal hydraulics. Using the code ATHLET, which is being developed by Gesellschaft für Anlagen- und Reaktorsicherheit, pre- and posttest calculations were done to determine the thermal-hydraulic events to be expected and to define and tune the boundary conditions of the test. The conditions found for natural circulation instabilities and cold-leg loop-seal clearing could be confirmed by the tests. The main thermal-hydraulic phenomena were identified and compared with the results gained during similar experiments on VVER-440 test facilities. Besides the thermal-hydraulic standard measuring system, the facility was equipped with needle-shaped conductivity probes for measuring the local void fractions.