Ground source heat pumps (GSHPs), also known as geothermal heat pumps, are a promising new technology that has been used for the last three years in the Turkish market. The main objective of the present study, which was performed for the first time in Turkey at the university level, is to investigate the performance characteristics of a GSHP system with a 50 in vertical 1(1)/(4) in. nominal diameter U-bend ground heat exchanger. This system was installed in a 65 m(2) room in the Solar Energy Institute, Ege University, Izmir (568 degree days cooling, base: 22 degreesC, 1226 degree days heating, base: 18 degreesC), Turkey. The Institute, built in 1986, has a livable floor area of 3000 m(2) and uses passive solar techniques. The heating and cooling loads of the room were 3.8 and 4.2 kW at design conditions, respectively. The system was commissioned in May 2000 and performance tests have been conducted since then. Based upon the measurements made in the heating mode, the heat extraction rate from the soil, with an average thermal diffusivity of 0.00375 m(2)/h, was found to be, on average, 11 W/m of bore depth, while the required borehole length in meter per kW of heating capacity was obtained as 14.7. The entering water temperature to the unit ranged from 5.5 to 13.2 degreesC, with an average value of 8 degreesC. The heating coefficient of performance of the heat pump and the whole system was extremely low when compared to other heat pumps operating under conditions at or near design values. The primary reasons for this were discussed in detail, and the potential for performance improvements was also suggested.