Under two combinations of roof and side vent openings of a semiarid greenhouse located in Tucson, Arizona, U.S.A, air and leaf temperatures, relative humidity, ventilation rate and water use (fog injection and plant water uptake) were measured. The natural ventilation rate was continuously measured by using SF6 as the tracer gas. Plant water uptake rate was measured using a sap flow gage. When the roof and side vents open, the inside temperature and ventilation rate were larger than when only the roof vent open. The inside air temperature increased with increasing ventilation rate. Converse responses were found for inside relative humidity and leaf temperature. Therefore, it follows that the vertical distribution of inside air temperature and relative humidity were increased with increasing ventilation rate. Water use in the greenhouse was reduced with decreasing ventilation rate and increasing internal relative humidity. Future work will be to develop the fog and vent control method to make air temperature and relative humidity uniform in the greenhouse and to reduce water use.