The main objective of the present study is to investigate the performance characteristics of a solar-assisted ground-source heat pump system (SAGSHPS) for greenhouse heating with a 50 m vertical 1(1)/(4) in nominal diameter U-bend ground heat exchanger. This system was designed and installed in the Solar Energy Institute, Ege University, Izmir (568 degree days cooling, base: 22degreesC, 1226 degree days heating, base: 18degreesC), Turkey. Based upon the measurements made in the heating mode, the heat extraction rate from the soil is found to be, on average, 54.08 Wm(-1) of bore depth, while the required borehole length in meter per kW of heating capacity is obtained as 12.57. The entering water temperature to the unit ranges from 8.2 to 16.2degreesC, with an average value of 9.1degreesC. The greenhouse air is at a maximum day temperature of 25degreesC and night temperature of 14degreesC with a relative humidity of 40%. The heating coefficient of performance of the heat PUMP (COPHP) is about 2.13 at the end of a cloudy day, while it is about 2.84 at the end of sunny day and fluctuates between these values in other times. The COP values for the whole system are also obtained to be 5-15% lower than COPHP. Copyright (C) 2004 John Wiley Sons, Ltd.