Effects of a high pressure (7 MPa) fogging system on inside dry-bulb (or air) and wet-bulb temperatures, relative humidity and ventilation rate as functions of vent configuration and weather conditions in a naturally ventilated greenhouse were studied experimentally during the summer. Two combinations of roof and side vent openings were examined with only the roof vents fully open, or both roof and side vents fully open. Two cooling methods were examined, namely natural ventilation (NV) and fogging combined with natural ventilation (FNV). A fog system was operated cyclically with a temperature difference between the inside dry-bulb and wet-bulb temperatures, when over 1°C it was operated, and when under 0.5°C it was stopped at 1 min intervals. The measurements were carried out with a tomato crop when 2.0 plants m-2 density was achieved in the greenhouse. The natural ventilation rate was continuously measured with sulphur hexafluoride (SF6) as the tracer gas. When the roof and side vents were open, the ventilation rate was larger than when only the roof vents were open. The inside dry-bulb and wet-bulb temperatures were decreased with increasing ventilation rate. At all fogging times, the inside dry-bulb temperature sharply decreased, while the inside relative humidity sharply increased. During the experiment, the differences between inside and outside dry-bulb temperatures, inside relative humidity and ventilation rate were 2.9°C, 55.2% and 3.6 m3 m-2 min-1 in NV, and -2.9°C, 88.2% and 2.3 m3 m-2 min-1 in FNV. Future work will develop a fogging and vent control method to reduce inside dry-bulb and relative humidity fluctuations in the greenhouse.