The Distributed Fibre-Optic Temperature-Sensing Technique (DTS) represents a new physical approach for temperature measurements in the geosciences and environmental sciences. It is based on optical time-domain reflectometry (OTDR). When a laser pulse is coupled into an optical fibre, light is backscattered as the pulse propagates through the fibre. The intensity and spectral composition of the backscattered light are determined by the molecules in the optical fibre. The Raman backscattering component is caused by thermally influenced molecular vibrations. Thus, its intensity depends on temperature. Because the velocity of light propagation in an optical fibre is well known, the distance of small backscattering intervals of the fibre can be determined from the travel time of the backscattered light. Thus, an optical fibre can be used as a distributed temperature sensor that gives temperature and distance simultaneously for the entire length of the optical fibre In the Grimsel rock laboratory of NAGRA, fluid-logging experiments in inclined boreholes 40 m deep were performed to test the capabilities of this method. For this purpose, an injection tubing was lowered to the bottom of the borehole, the optical temperature-sensing cable being clamped as a fixed sensor array at the rod. Warm and cold water were injected and temperature profiles were recorded every 1 min. The propagation of the temperature front along the borehole allowed us to determine the location of water-bearing fractures and to estimate the flow rates. These experiments showed that the fibre-optic temperature-sensing technique meets the requirements of fluid-logging experiments. The technique is easy to handle and can be used under field conditions. Transient effects are caused by different injection and extraction regimes.